Epistemic Crises: Their Origins and Their Resolutions

Three Case Studies

Larry Laudan

8Larry Laudan 2002

Contents

Acknowledgments ........................................................................................ iii Prologue ................................................................................................................... 1 The Conventional Wisdom about Epistemic Crises ............................................................................................................................................. 2

Proving Guilt in the Law ................. Error! Bookmark not defined. The Backdrop: Trial by Ordeal ......................... Error! Bookmark not defined. The Parting of the Ways ..................................... Error! Bookmark not defined. The Revival of Roman Law ................................ Error! Bookmark not defined. The Curious English Experiment ..................... Error! Bookmark not defined. The Crisis of the Jury and Its Weighing of the Evidence ... Error! Bookmark not defined. Guilt Beyond a Reasonable Doubt ................... Error! Bookmark not defined.

Postscript .............................................................. Error! Bookmark not defined. Torture, Once Again ............................................ Error! Bookmark not defined. Conclusion ............................................................. Error! Bookmark not defined.

Astronomy=s Struggles for Legitimacy and Ascendancy ........................................................................................................... 8

The Science-Knowledge Equation .......................................................................... 9 The Ancient Version of the Crisis of Astronomy ............................................... 11 The Medieval Interlude ............................................................................................ 17 The Astronomical Revolution, Properly Speaking .......................................... 19 The Invention of Moral Certainty ......................................................................... 26 Astronomy=s Revenge ............................................................................................. 30

Conclusion .................................................................................................................. 31

The Crisis of Cosmogony, 1770-1900 Error! Bookmark not defined.

Introduction .......................................................... Error! Bookmark not defined. The Background: The State of Cosmogony cerca 1800 ...... Error! Bookmark not defined. The Critique of Cosmogony ............................... Error! Bookmark not defined. Cosmogony=s Revival ......................................... Error! Bookmark not defined.

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Acknowledgments [to be written]

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Prologue For the last quarter century, I have been thinking and writing about what I here call epistemic crises. Progress and Its Problems (1977) was my first foray in print into this area, although I had been musing on their nature since I first studied with Thomas Kuhn in the early 1960s. Those of us who have written about such events -and I include here Kuhn, Stephen Toulmin, Paul Feyerabend, Imre Lakatos, and myself among others- have sought to make our philosophisizing appear honest toil by alluding to hordes of historical examples and illustrations that supposedly bear out our respective analyses. (By my count, Kuhn refers to more than 200 historical episodes in his Structure of Scientific Revolutions.) This cavalier use of history -frequently alluding to three or four episodes in every paragraph- lends one=s writing an initial air of authority by suggesting a mastery of large amounts of historical material. Yet among those of us who engaged in such rapid-fire conjury of illustration after illustration, there was always the lingering worry that a detailed analysis of some of the cases cited with such abandon might not in fact sustain the heavy philosophical load to which they were being subjected.

Unable to speak for the others, I for one have long hoped to be able to leave such wholesale illustration-mongering to one side and to offer, instead, a detailed analysis of a few examples of intellectual crises and revolutions to see what light such instances might shed on our general theories about such events. This book is the fruit of some free time I have finally found to do precisely that.

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1 The Conventional Wisdom about Epistemic Crises My focus in this book is on changes of epistemic standards, changes that are typically induced by a broad perception in the relevant community of an epistemic crisis. But what, precisely, is that? The full answer, or at least a fuller answer, to that question is what this brief study aims to provide. Being too explicit at this stage would preclude the open-ended investigation into how such crises function of the sort that this book aims to exemplify. Still, to a crude, first approximation, simply to get the conversation started, we can say that an epistemic crisis arises when one of the following situations occurs:

-researchers in some field of investigation begin to suspect that the criteria, principles, and rules of thumb that they have been using for establishing claims to knowledge (and this can include everything from explicit criteria of theory evaluation, to rules for the design of experiments and trials, to the very aims of inquiry) are unfounded;

-such suspicion about the relevant standards leads inevitably to doubt about the theories and other beliefs that those standards have aided in selecting, that is, general skepticism is apt to result;

-theorists in the relevant field(s) of investigation cast about trying to figure out which elements of the existing epistemic Apackage@ to jettison or modify and, inevitably, they explore alternative accounts of knowledge and rational belief.

Sometimes a crisis is quickly resolved by tweaking or adjustment of the standards already in place. When this occurs, the crisis itself is usually quickly forgotten by everyone except historians. Other times, however, no jiggering around with the current machinery will provide a strong riposte to the concerns that initiated the crisis in the first place. When this happens, there are generally only two responses, both radical. One involves producing a new and more plausible set of standards that enable the community to preserve all, or at least some, of the beliefs picked out by their previous standards as sound. Failing that, and this is the second response, skepticism in the discipline in question becomes endemic. As often as not (consider the cases of astrology or phrenology), the epistemic community dissolves -save for a few true believers who persist with the traditional practices even after they have lost their intellectual rationale.

Imagine a simple hypothetical example: an isolated tribe of natives has been wont to decide the future by consulting the local witch doctor for advice. His procedure has been to cast 11 stones from a leather pouch onto the table. Each stone has two sides, one painted red, the other white. If more of the stones turn up white, then the signs are auspicious; otherwise, they portend danger. Up to now, villagers have been consulting the stones for deciding whether to take a journey down river by canoe today or to postpone it, whether to organize a new hunt, whether to plant crops this month or next

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month, where to drill for water, and so on. A missionary arrives in the village one day and sees the villagers consulting the wiseman. Eager to impress the locals (and to discredit her rival), she goes to him to seek his advice on every aspect of her life. Each piece of advice she receives, she ignores. His stones say she mustn=t start construction on her church this week, so she does. They say that using river transport for bringing in her hymnals by barge this season would be dangerous, so she arranges the shipment immediately. And so on.

Initially, of course, the villagers think that she is crazy, ignoring, as she is, all the signs and portents. Nevertheless, in time they conclude that she is none the worse for that. Her church is now completed. She has suffered no apparent ill effects from proceeding brazenly against the advice of the stone caster. Soon, several members of the tribe stop consulting the stones before they embark on important ventures. Some of their ventures work out, others are failures, but the skeptical villagers are no worse off than the rest. As this pattern repeats itself, the villagers--including the witch doctor himself- begin to wonder whether the stones are as good as they used to think they were about predicting the future.

Here we have all the makings of an epistemic crisis. Traditional tools for the fixation of belief no longer inspire the confidence (and the awe) they once did. Perhaps the tradition of consulting the stones will pass into oblivion or, if the stone caster is lucky, he can salvage his reputation. Whether he triumphs in the end or not, the villagers have gone through an epistemic crisis.

This book focuses on the conception of an epistemic crisis, a notion that turns out to be a good deal more complex than our simple and contrived example might lead one to think. In this first chapter, I will briefly survey some prevalent theories about such creatures, turning in the rest of the book to see what we can learn by looking closely at a few specimens. Although scholars disagree about precisely what an epistemic crisis is, a few things can be said uncontroversially, by way of introduction. For one thing, epistemic crises are crises of authority, but authority of a particular sort. They arise when the traditional certifier of beliefs, the doxastic authority--whoever or whatever that happens to be-- comes under suspicion for being a less than reliable source of beliefs and an untrustworthy ground for the actions based on those beliefs. Almost everyone has experienced individual epistemic crises: perhaps the discovery in childhood that parents and teachers were not always correct, or the discovery that religious authority often gets things wrong. This book is not about individuals going through such crises, but will focus rather on what happens when entire disciplines, communities, cultures, or traditions pass through such a traumatic event.

Not every crisis of belief is an epistemic crisis. Sometimes we discover that a belief we have long held no longer works, perhaps because it was dramatically refuted or otherwise failed dismally. We know that we have to cast around for a new belief to replace the old one, but our implicit epistemology often remains intact (especially if it happens to be an epistemology that admits its own occasional fallibility). This is precisely what happened to physicists when Einstein=s theory of relativity was confirmed. They were obliged to give up many cherished Newtonian beliefs about the nature of space and time but they were not driven to question whether their core belief-

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authenticating mechanisms, the empirical methods of physics, were legitimate. Indeed, the most frequent pattern, when there is a crisis of belief, is to use the already in-place epistemology to help us decide what to believe when our older beliefs no longer serve. Accordingly, most crises of belief are not epistemic crises, in that they do not lead us to doubt our basic belief-certifying processes. To say as much is already to acknowledge how acute a genuine epistemic crisis will be when it does occur, for such a crisis challenges the very tools that we have been using for determining what to believe. During an epistemic crisis, nearly all our relevant beliefs are in doubt because we have begun to doubt the very rules that have until now justified those beliefs.

Yet the graveness of the problems caused by an epistemic crisis goes beyond casting doubts on everything we have previously believed. The nub of the problem is that, having once decided that the previous epistemic regime is no longer reliable, we have to find a new set of rules for fixing beliefs. How are those to be settled on? Put so abstractly, it appears an irresolvable conundrum. If we think of an epistemology as like the rules of a game (here the deadly serious game of authenticating our beliefs), an epistemic crisis is akin to discovering that the rules of a familiar game don=t work any longer. How, in such circumstances, do we settle on new rules? Need we infer that the new rules are inherently arbitrary, as the rules of games are often thought to be?

The notion of an epistemic crisis is not a new one. For at least half a century, scholars of various persuasions have been using the idea as a tool for understanding various important, sometimes wrenching, historical changes. Probably the three best-known accounts of the general structure of epistemic crisis are associated respectively with the work of Michel Foucault, Thomas Kuhn, and Alasdair MacIntyre. All three fastened on the idea of an epistemic crisis to characterize a specific type of intellectual event that occurs from time to time in the history of many disciplines, communities, and traditions.

Kuhn and Foucault believed that the answer to our question whether the adoption of new epistemic rules is arbitrary second question should be affirmative. Recall that for Kuhn a change of paradigm involves not only new beliefs about the world, but also new principles (he calls them Astandards@) governing what we should believe:

In learning a paradigm the scientist acquires theory, methods, and standards together, usually in an inextricable mix. Therefore when paradigms change, there are usually significant shifts in the criteria determining the legitimacy both of problems and of proposed solutions.1

When the old, reigning paradigm is threatened, a Aperiod of crisis@ follows, during which intellectuals ponder replacing it with an alternative paradigm, one that involves both new beliefs about the world and new standards. Once a rival is found, the choice between the old paradigm and its would-be successor(s) takes place outside the usual rules of the game, since-according to Kuhn--the core beliefs of each paradigm look strong by its own standards and weak by the standard(s) of its rival(s). Mutual incomprehension reigns. Advocates on each side of the divide Afail to make complete

contact with each other=s viewpoints.@ Choice of paradigm and choice of standard must be ultimately subjective and arbitrary in such circumstances. Notoriously, Kuhn insists

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that the acceptance of a new set of standards is a Aconversion experience@ rather than a deliberative act.

Kuhn thinks further that communities, at least scientific communities, have a low tolerance for the sort of near-anarchy that is often characteristic of epistemic crises; accordingly, he maintains that crises are invariably short-lived events, punctuating much longer periods of Anormalcy,@ where nearly all the members of the relevant community agree on the appropriate standards to use.

If you prefer the Gallic version of matters intellectual to the Anglo-Saxon one, you can find much the same family of notions in Foucault=s doctrine of an Aepisteme.@2 Foucault was struck by the fact that from time to time, the implicit and explicit epistemic standards (what he called the Aepisteme@) of a community abruptly

underwent a mighty shift, a Arupture.@ A new episteme ushers in the possibility of new forms of knowledge, new forms of science, and new forms of discourse, while it simultaneously closes off or renders unintelligible older forms of discourse and thought. When such a change occurs, those working within the later episteme are literally incapable of understanding or communicating with those still wedded to the former. Such shifts of episteme are obviously profound and far-reaching events, and (like Kuhn=s paradigm shifts) they are irrational upheavals. No plausible and compelling story can be given about why the victor triumphed or why the vanquished lost.

Even with only these thumbnail sketches before us, we can already see that we must be alert to several things in examining the two crises that form the case studies of this book. We will want to ask, for instance: $ Are epistemic crises generally brief interludes in an otherwise crisis-free history? $ Are epistemic crises always associated with changes in all the other elements of

the belief complex of a community or a discipline? $ Do those on opposite sides of an epistemic crisis fail to grasp the issues at stake,

always or generally talking past one another? $ Is the adoption of a new epistemic regime, following a crisis, irrational and

subjective? I said that MacIntyre has also had much to say about epistemic crises. In brief,

what his analysis adds to this mixture is a stress on continuity and tradition from one paradigm to its successor. His principal argument is that a new paradigm or episteme cannot succeed unless its advocates can construct what he calls a narrative, a historical account that ties together past and present into a coherent tale of progress and development. The resolution of an epistemic crisis requires, in his words, Athe construction of a new narrative which enables the agent to understand both how he or she could have held his or her original beliefs and how he or she could have been so drastically misled by them.@3 On this account, the abrupt ruptures of Kuhn and Foucault give way to a very different picture in which the success and the rationality of a later paradigm depend upon its being able both to explain why its predecessor once seemed so plausible and why, despite that plausibility, it ultimately failed. What establishes the rationality of the replacement of one paradigm or set of epistemic standards by another, according to MacIntyre, is the ability of the successor to characterize the transition from the one paradigm to the other as a perfectly natural progression from a good way of

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doing things to a better way. For him, changes of paradigm or episteme are not the Aruptures@ that Kuhn and Foucault took them to be. He denies the radical incommensurability between successive epistemic regimes (to pick a term neutral as between paradigms and epistemes) that Kuhn and Foucault took as fundamental. For our purposes, the chief question we will need to ask of our case studies, relative to MacIntyre=s analysis, is simply this: $ Do the proponents of new and successful epistemologies have the resources to

construct the inclusive, face-saving narratives for their predecessors that MacIntyre regards as so crucial? Apart from Kuhn, Foucault and MacIntyre, the views of one further thinker

should be mentioned, if only as a warning. I refer to Karl Popper. Both he and his many disciples tend to the view that epistemic crises, at least as understood here, are nonexistent. Their position is that, as a species, we humans are all more or less hardwired to accept the same epistemic rules and principles. Popperians grant that intellectuals can and often do disagree about their substantive beliefs about the world. However, when it comes to standards, they hold that these are not up for negotiation and are never subject to genuine debate.

When Popperians encounter apparent cases of epistemic crisis, that is, of thinkers who question prevailing epistemic norms, they regard the resulting dispute as a largely verbal affair with no real substance, a mere querrelle de mots, since deep down we and our fellow creatures share the same cognitive apparatus and the same epistemic instincts. Little more will be said directly in this volume about the Popperians (apart from urging them to look and see), but it is important for the record to stress that many contemporary thinkers do not share the author=s view that epistemic crises are real. I invite such readers to regard this book as a test of the hypothesis that disagreements and debates about epistemic standards do much to drive some of the most important changes of belief and practice. If, as they insist, there are no important divergences about epistemic standards within and between disciplines and traditions, then it should be impossible to construct the sort of stories that I try to tell. If, as I hope, these stories appear more than a little plausible, it falls to them to devise alternative narratives for the events described here, narratives that do not hang their central story line on epistemic disagreement and crisis.

Of course, much, much more needs to be said about epistemic crises than I have sketched here. Indeed, more will be said in chapter five but to spell out the idea any further at this point would be to jump the gun. The motive for exploring the detailed case studies of the next three chapters is to give us ample fodder for speculating about how epistemic crises arise and how they are resolved.

Before getting down to the details, a brief explanation is in order as to the choice of case studies included here. There is a wealth of possible examples that could have been chosen, ranging from the crisis in 19th-century Protestant thinking in the wake of

Darwin s discoveries to the crisis in contemporary psychoanalytic thought in the face of the failure of Freud=s proposals for the clinical validation of psychoanalytic diagnoses. Rather than select such timely topics--where we are still very much in media res, and where the crisis is far from over--I have decided to dredge further back in the historical

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record in order to focus on epistemic crises that began, went on, and ended definitively. My examples come from the criminal law, from astronomy, and from cosmogony and span the time spectrum from antiquity to the beginning of the 20th century. I have tried to be mindful of the need to select examples as different from one another as

I could make them, so as to avoid whatever bias might creep in by limiting one s examples entirely to the natural sciences or to one epoch in history. Another reason for the seemingly idiosyncratic pairing of examples from science and the law is that both these intellectual traditions, and their associated institutions, are widely conceived as epistemological engines, whose primary function is to crank out true beliefs--either about the guilt of an accused, about the structure of the heavens, or about the history of the cosmos. Beyond that, these three cases are fascinating in their own right as examples of the kind of high drama that intellectual life and academic debate can sometimes provoke. My method will be to describe each largely in its own terms in the next three chapters, leaving the final chapter for some philosophizing about what these cases might lead us to conclude about the strengths and weaknesses of the received view of epistemic crises.

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3 Astronomy=s Struggles for Legitimacy and Ascendancy

Despite Kepler and Galileo, we believe today, with Osiander and Bellarmine, that the hypotheses of physics are mere mathematical contrivances devised for the purpose of saving the phenomena. -Pierre Duhem (1908)4

Astronomy, we like to imagine, is one of the oldest sciences. Remains of proto-observatories dot the globe, according to archeologists. Cultures as diverse as the Chinese, the Egyptians, the Mayans, and the Druids all had calendars based on careful observations of changes in the heavens. Our astronomy, that of the West, is generally dated from the Babylonians, who accumulated several hundred years= worth of data about the motions of the planets, the Sun and the Moon, data that were to proof invaluable to Greek astronomers up to and including the great Claudius Ptolemy in the first century A.D. Along with Euclid=s Geometry, the Almagest of Ptolemy is nowadays regarded as the most important and influential scientific work of the ancient world. I rehearse these familiar facts because they serve to underscore how closely astronomy sits to center stage in our conception of the early story of scientific progress.

It may, therefore, come as something of a surprise to realize that, through most of the last two millennia, astronomy=s status as a legitimate, let alone as a genuine and important, scientific subject has been much in dispute. This second case study will focus on the shifting epistemic fortunes of astronomy from antiquity to the early modern era when, finally, mathematical astronomy gained acceptance as a genuine science, at the very heart of the Scientific Revolution of the 17th-century. Astronomy=s historical vicissitudes in this period mirror a broader series of crises about the nature of science and knowledge in the West that touched most of what we now call the sciences, including areas far afield from astronomy itself, such as medicine and mathematics. I will focus chiefly on astronomy, however, because its evolution exhibits those crises in a particularly stark and salient fashion. But before moving straightaway to the heavens, our study must begin, for reasons to become clear shortly, with a topic much less sublime, namely, early Greek medicine.

By the 5th century, B.C., medicine in Greece had already marked itself off from the healing arts practiced in most other cultures by adopting two postulates: One was a naturalistic premise to the effect that diseases were not to be understood as mysterious visitations from the gods, to be cured by exorcisms or

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prayers, but as imbalances due to perfectly natural and ordinary causes, such as diet, exercise, bad water, freak weather, and the like. The second key postulate of Greek medicine, that of empiricism-- associated especially with the school of physicians around Hippocrates--insisted that all medical doctrine was to be grounded in, and scrupulously checked against, the experience of the patient and his physician. Because of this second doctrine, the followers of Hippocrates quickly became known as Aempirics,@ or (as we should say) empiricists.

Hippocrates= opponents were the Adogmatists,@ such as Empedocles, who thought that medical knowledge should begin from certain grand theoretical posits (such as atoms, or elements, or basic qualities) and proceed, not by the Hippocratic method of generalization and induction from detailed case histories of patients, but rather by a firm grasp on the underlying causes and principles of life and disease. Hippocrates and his school cast themselves as defenders of the empirical tradition in medicine against these theoretical interlopers who were keen to integrate medicine within a broader framework of physics and chemistry.

By the fourth century, B.C., the Hippocratic school was facing an even stiffer challenge. Plato and Aristotle had joined the battle on the side of the grand theorists, arguing that all genuine science, medicine included, must be grounded on self-evident first principles. Indeed, it was the upshot of their analysis that medicine, if done in Hippocratic fashion, could not be considered as a science at all but simply as a version of superstition. The Science-Knowledge Equation Both Plato and Aristotle articulated a sharp distinction between knowledge and mere opinion (opinion, in those days, was almost always Amere@). Knowledge was said to consist of those propositions that we know to be true, that could not be otherwise, that are fully certain. Opinion, by contrast, was more or less all the rest of our beliefs. It included everything from completely ill-founded speculation to beliefs that were generally reliable but still fell short of certainty. Plato and (especially) Aristotle (384-322 B.C.) appropriated the knowledge-opinion dichotomy as a tool for defining what could be genuinely regarded as a science. In a word, science was knowledge. Or better, science was a special sort of knowledge, one that was causal and theoretical, that involved finding the essences of natural objects and then deducing from those essences propositions about the detailed behavior of things. Because there were certain types of knowledge that were not part of genuine science (for instance, various forms of practical knowledge), we must resist any identification of the two. But this much may be said without qualification: there was, for Aristotle, no authentic science that was not a part of knowledge.

This doctrine, developed at length in his Posterior Analytics, entailed that where there was lack of certainty (and thus mere opinion), there could be no knowledge and hence no science. To say that Aristotle=s view of things was enormously influential is vastly to understate the case. For almost two millennia,

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the prevailing view of the nature of science involved this Peripatetic insistence that science is knowledge and therefore certain and infallible. (Even now, in vestigial testimony to the onetime ubiquity of this notion, we speak of Ascientific

knowledge@ rather than Ascientific opinion,@ despite the fact that most of the central doctrines of the sciences are clearly opinion rather than knowledge.)

It is universally agreed that the philosophers=s paradigm science (for Plato as much as for Aristotle) was that of geometry. It was, as we would now say, an axiomatic system, whose axioms or first principles were thought to be self-evident. The geometer deduced theorems from these axioms and thus developed a comprehensive account of the properties of lines, planes, and solids. Given that the axioms of the system were self-evident, all the theorems were likewise certain, since deduction was truth-preserving. No one who understood the axioms of geometry could fail to perceive, on due reflection, the truth of the theorems of the system.

From our perspective, of course, the choice of geometry as the paradigm science is more than a little curious since there is no room for consulting experience, let alone doing experiments, in geometry--save possibly at the beginning when it comes to evaluating the truth of the axioms. For us, geometry is scarcely a science at all, precisely because it fails to require that its conclusions be checked against experience. It doubtless is a form of knowledge, but science it is not, understanding (at least as speakers of English do) that being scientific requires the empirical confirmation of one=s hypotheses. Greek philosophers saw things almost exactly in the reverse fashion. They did not say that the empirical was unscientific per se but, as we will see in detail shortly, they were inclined to suppose that empirical checking of theoretical doctrines was inevitably inconclusive and thereby incapable of producing anything at the theoretical level except opinion.

While the immediate target of Aristotle=s critique of opinion masquerading as science were the practitioners of medical empiricism, it should be clear that his analysis of what was necessary for an activity to be genuinely scientific likewise had profound implications for astronomy. Like the physicians, most early astronomers had been empiricists of a sort, carefully cataloging the movements of heavenly bodies and attempting to find in those motions some general patterns. The astronomers no more began their craft with self-evident first principles than the physicians did and the astronomer was almost as far removed from Aristotle=s privileged science, geometry, as the doctor was.

Aristotle thought he could change all that, at least as far as the science of the superlunary regions were concerned. To that end, he tried his hand at constructing an astronomy that would be genuinely scientific by his own criteria. Specifically, in his book on the heavens, De Caelo, he developed the ideas of his contemporaries Eudoxus and Callipsus into a very elaborate model of the heavens, generally known as the theory of homocentric spheres. (See diagram 1.) According to this model, the Earth sits motionlessly at the center of the heavens while everything revolves in uniform circles about the Earth. The fixed stars, and

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everything else, complete one revolution every 24 hours, producing the familiar diurnal motion of the heavens. Beyond that, each of the planets (including the Sun and the Moon) engages in more elaborate motions, still uniform and circular, that compound with the diurnal motion to produce the cosmic dance that we see in the night sky. Very roughly, the planets (but not the fixed stars) have a west-to-east motion along the ecliptic, as well as a universal east-to-west diurnal revolution. The speed of this contrary motion is not the same for all of the planets. This explains why different planets take different amounts of time to return to same point on the celestial sphere. According to this model, the observed motions and positions of heavenly bodies are compounded out of these individual motions, each of the latter being both geocentric and uniform.

Aristotle=s arguments for the theoretical postulates he makes are generally a priori and metaphysical: Heavenly bodies move in circles because it is the essence of the heavenly stuff so to move. Circular orbits have neither beginning nor end and are thus eternal. The motion of the heavenly bodies must be uniform because any change in their (orbital) speed would require the intervention of some other object or force and that is not conceivable in a changeless heaven.

To a rough, first approximation, the astronomy of Aristotle functioned well enough. It could, for instance, explain such subtleties as retrograde motion, when a planet briefly appears to double back on its own path. It likewise made sense of solar and lunar eclipses and the phases of the Moon. But it was far from flawless. Most prominently, it could not explain, and appeared to be flatly refuted by, the fact that both the brightness and the apparent size of many heavenly bodies change periodically. (For instance, Venus and Mars sometimes appear five to six times as bright and as large as at other times.) Those phenomena seem hard to square with the Aristotelian postulate that such planets always remain at a constant distance from the Earth, especially if we combine it with another familiar Aristotelian dogma to the effect that, besides motion, there are no sources of change in the heavens.

These empirical anomalies notwithstanding, Aristotle=s theory of the cosmos won many adherents, not least because it seemed to be grounded, as geometry, on a set of self-evident truths, in this case truths about heavenly bodies rather than points and lines. By the beginning of the Christian era, the received Ascientific@ picture of the cosmos--except among working astronomers- was that it was both geostatic and geocentric. That exception, the handful of professional astronomers, proved to be the source of the crisis we will examine. The Ancient Version of the Crisis of Astronomy It was not long after Aristotle developed his model of the heavens before serious astronomers concluded that Aristotelian cosmology -at least insofar as it involved homocentric spheres--simply would not do.5 The evidence that planets change their distance from the Earth, evidence derived especially from observations of

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their size and brightness, made Aristotle=s constant-distance-to-the-Earth postulate simply unacceptable to anyone concerned to Asave the observable

appearances,@ as they were wont to phrase it. Accordingly, astronomers began exploring models that might offer the resources for explaining such changes in size and brightness. Above all, these models would have to postulate a variable earth-to-planet distance in order to accommodate the relevant anomalies.

Most prominent here were the eccentric and the epicycle models. Put simply, a planet moving along an eccentric was revolving uniformly about a point that was not the center of the Earth. With such a motion, the planet would obviously sometimes be closer and sometimes farther away from us, thus explaining changes in brightness and apparent size. In the epicyclic model, the planet would be carried uniformly around a circle (the epicycle) whose center was itself revolving uniformly around the Earth. This system too would allow the planets=s distance (and thus their apparent sizes and brightnesses) to vary significantly. Even more exotic was the equant, utilized extensively by Claudius Ptolemy (fl. 2d cent. A.D.). Here, the planet revolves in a circle about a point different from the Earth (like an eccentric) but its motion is not uniform with respect to either its center of revolution or to the Earth. Rather, there is another point, the so-called equant point; the planet=s motion is uniform only in the sense that, relative to the equant point, it sweeps out equal angles in equal times. (Needless so say, this is not what Aristotle and Plato had in mind when they insisted that heavenly motions be Auniform.@) [LL: diagrams of the three mechanisms]

Quite clearly, these models of the astronomers disagreed with the geometry Aristotle had attributed to the heavens: homocentric spheres had been laid completely to one side. Still worse, they violated certain core principles of Aristotelian physics regarded as self-evidently true, such as the axiom that the all heavenly bodies moved about the center of the Earth, and the axiom that heavenly motions were uniform with respect to their center of revolution. If these plausible postulates of Aristotle=s physics were right, the newly emerging astronomical theories simply had to be wrong. The nagging worry was that the latter fit the facts, saved the phenomena, far better than any known system built on Aristotelian foundations seemed able to. This conflict between the physics of Aristotle and the physics implicit in the models of the astronomers was one part of the epistemic crisis that was to dog astronomy for the next millennium and more.

But the problem facing the astronomers was not only a clash between a plausible Aristotelian physics on the one side and an empirically powerful astronomy on the other. If one accepted the models of the astronomers, one was in fact and in effect repudiating the very notion of scientific knowledge that Plato, Aristotle and other Afoundationalists@ had worked so hard to articulate and defend. This is because astronomy, done as such professional astronomers as Ptolemy wanted to do it, did not begin from axioms known to be true. On the contrary, it began with postulates that were, in many cases, implausible,

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arbitrary, and theoretically unmotivated. Still worse, the astronomers could not derive the motions of the heavens from their core axioms. They had to constantly consult observation and jigger around with the relevant parameters until they were able to reconstruct the motions of the heavens. Their models of motion were thus a congeries of core assumptions (themselves not all self-evident) and other doctrines opportunistically invented to handle some anomaly or other. I have already mentioned the problem posed by the equant, a core assumption that utterly lacked theoretical rationale.

An equally acute problem of the second, parameter-fitting sort will illustrate the conundrum. In designing his models for the Aouter@ planets (Mars, Jupiter, and Saturn)--said to be outer because they were beyond the Sun in the ancient scheme of things--Ptolemy adopted a curious assumption. It supposes that the line drawn from an outer planet on the circumference of its epicycle to the center of that epicycle was always parallel to the moving line between the Earth and the Sun. (Technically put, that the radius vector of the planet to its epicycle center always moved as the radius vector between the Sun and the Earth.) [LL: diagram]

There was no conceivable theoretical or physical motivation for this assumption in a geostatic astronomy such as Ptolemy=s. Quite the reverse, it cries out for some theoretical justification as to why there should be any connection between a planet=s motion around its epicycle and the motion of the Sun relative to the Earth. Ptolemy had none to offer. Still worse, he lacked any direct empirical evidence for this presumed connection. He could not observe the location of the center of an epicycle of an outer planet and note the location of the planet relative to that center. But neither could he dispense with this postulate. Without it, he could not fix the parameters of the model of any outer planet.

Yet--thought the philosophically orthodox--what sort of Ascience@ can it be whose assumptions (such as this) are themselves adopted without rhyme or reason? This premise of a solar component to the motion of the planets is about as non-self-evident as any principle one can imagine. One might have expected that the response from the astronomers to this serious challenge would be something along these lines: it is true that our core postulates have no initial justification, but they acquire their plausibility, their scientificity, from their evident success in explaining and predicting the heavenly motions. That is, one would expect them to emphasize the empirical credentials of their science in the face of challenges to its theoretical coherence, saying (as Newton would sixteen hundred years later, when challenged as to the coherence of his notion of gravity): Athese are the facts, whether we have a theoretical understanding of

them or not.@ Unfortunately, this response was not open to the astronomers for two

quite distinct reasons. One of them had to do with the Greek understanding of the nature of logic and inference. The Skeptics aside, the Greeks generally had no quarrel with the claim that one could use experience to establish the truth of specific, observable claims about the world (e.g., Athis planet is brighter than that

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one@) or even to establish general laws of nature (e.g., AAll crows are black@). They were not proto-Humeans, skeptical of every inference from a specific instance to its generalization. On the contrary, it was in precisely this manner that Aristotle believed that the first principles or axioms of a science were established, via the mind grasping Athe universal that inheres in the

particulars.@ What did bother Greek logicians a great deal, however, was any argument that had the structure:

If this hypothesis H, dealing with unseen things, is true, then we should expect to observe O.

We do observe O.

Hence H is true.

This was universally recognized by Greek philosophers as a logical fallacy, which of course it still is. (Logicians call it the fallacy of affirming the consequent.) Now, Ptolemy=s theory was in trouble because any attempt to ground it empirically seemed to be an instance of this fallacious argument. Ptolemy might have said, for instance, that his controversial postulate of a solar component in the motion of the outer planets is established or made at least probable by the fact that such a postulate enables him to predict and explain the motions of such planets. Yet Ptolemy knew too much about ancient logic to suppose that he would be permitted to argue thus. Similar problems afflicted any Greek scientist who might seek to ground his theoretical speculations on post hoc, empirical confirmations. Showing that an hypothesis has true consequences, even many true consequences, does nothing to establish the truth of that hypothesis, since false theories can and do have many true consequences. Greek empiricism, as we might call it, was hobbled by this result. None of the ancient masters of empirical science -certainly neither Ptolemy nor Galen nor Hippocrates- had a convincing rejoinder to the claim that the post hoc confirmation of theoretical hypotheses was inevitably inconclusive and incapable of producing certain, scientific knowledge.

As if to underscore the acuteness of the problems associated with using confirmation as a test of the truth of hypotheses, Greek astronomers themselves made a series of discoveries that chipped away at any pretension of astronomy to scientific status, and those discoveries collectively provide the second reason why Greek astronomers could not take the simple empiricist way out. Specifically, Greek astronomers discovered that they were certain pairs of models of planetary motion that, although profoundly different in their geometry, were observationally indistinguishable. Consider but one example among several. [LL: diagram] Take, on the one hand, a standard epicyclic picture. Call the deferent radius r and the epicycle radius, R. The motion generated by a planet driven by these two motors will be exactly the same as that generated by a planet that

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moves in a circular path with diameter R around a point that is itself moving uniformly around the Earth at distance r. Dynamically, these are quite different pictures: in the first case, a planet=s primary orbit is around a stationary point,

the Earth. In the second, the planet=s primary (or larger) orbit is about a point that is itself revolving in space. Ptolemy and the other Greek astronomers who discovered these results insisted that there was no possible observation from the Earth that could distinguish between these two different models.6

But, if true, that is a body blow to the possibility of grounding astronomy on some form of empiricism. The latter doctrine, after all, insists that we must look to experience to decide what theories we should accept. But in examples of this sort, experience is apparently powerless in principle to drive our choice, since our observations would fit equally well with either model. This problem of the underdetermination of theory by observation seemed to foreclose any attempt to ground one=s choices of astronomical model on experience alone.

To make matters still worse, the empiricism of Ptolemy, like that of virtually every other ancient thinker except Claudius Galen, was both primitive and undiscriminating. Driven by the Platonic slogan Ato save the phenomena,@ it failed to distinguish between what a theory explained and what it predicted, between especially salient facts and largely insignificant ones, and between confirmations that were powerful and those that were weak. What it aimed to establish, rather, was a full and complete concordance between the consequences of a theory and what could be observed.

One particularly telling instance of this failure to grasp the probative force of certain sorts of data occurs in an intriguing discussion by Ptolemy in his cosmological tract, Planetary Hypotheses. There, he uses his theory of the nesting spheres of the planets in order to derive the minimum distance from the Earth to the Sun.7 He checks this theoretically derived value against the already known value of that distance (computed by Ptolemy=s predecessors on the basis of elaborate measurements made during solar eclipses) and discovers that his theoretical value (1,080 earth radii) differs by only about 7 percent from the independently ascertained value (1,160 e.r). Instead of regarding this (as we moderns would) as a stunning confirmation of his theory of nesting spheres, Ptolemy frets about the fact that the two values -the predicted and the observed- are not in perfect accord!8 He proceeds to propose some ad hoc adjustments of certain parameters of his model (especially the Earth-Moon distance) in hopes of reducing the discrepancy. The point is that what could have been touted as a striking empirical success of his system becomes, instead, a mildly embarrassing anomaly, to be explained away. With such friends, ancient empiricism needed no enemies.

By late antiquity, the status of astronomy had become thoroughly confused. Philosophers generally inclined to the view that the a posteriori methods of proof used by the mathematical astronomers were inherently inconclusive and that, accordingly, quantitative astronomy could not claim a legitimate place among the sciences. For their part, astronomers were apt to

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distinguish the cosmological part of their work from their mathematical models. Prepared to concede that at least some of the latter were indeed underdetermined by the evidence, they nonetheless insisted that they genuinely Aknew@ many things about the heavens beyond any reasonable doubt. This cosmological knowledge would have included the beliefs that the Earth is more or less in the center of the cosmos; that the cosmos, like the Earth itself, is a sphere; that the Moon and Sun are closer to the Earth than Jupiter and Saturn; that the source of the planets= illumination is reflected sunlight; that eclipses of the Moon are caused by the passing of the Earth between Sun and Moon; and scores of other doctrines. Beyond this, their mathematical models, scientific or not, had proved themselves remarkably good at predicting the movements of the stars, the timing of eclipses, and changes in brightness and apparent size of the planets.

What was at issue was whether these impressive achievements could justify astronomy=s pretensions to be a genuine science or whether, in the final analysis, empirical astronomy was simply a pseudoscience, perhaps full of useful rules of thumb but utterly lacking in conceptual coherence. The dilemma was obviously acute. Either astronomers had to abandon what they regarded as their best theories (because those theories didn=t exhibit the credentials demanded by the prevailing standards) or else they had to replace those epistemic standards themselves. The only even mildly promising alternative epistemology, some form of empiricism, was itself undermined by technical arguments mounted by the astronomers themselves.

Unfortunately, neither ancient astronomers nor ancient philosophers had the epistemological tools for resolving this conundrum. To the question, ADoes

empirical success constitute a criterion for being scientific?@ no one had any plausible, positive answers -although there were plenty of plausible denials. As we will see below, the defense of a theoretically rich, empirical science, replete with unobservable entities and invisible mechanisms, had to wait until the 17th century. In the meanwhile, it was a largely a priori physics and a purely a priori geometry that remained the paradigms for scientific knowledge throughout Late Antiquity, the Middle Ages, and the Renaissance. Euclid and Archimedes invariably took pride of place over Ptolemy in ancient catalogues of the sciences. Under such circumstances, Ptolemaic Astronomy--in retrospect the crowning scientific achievement of the ancient world- struggled with only modest success to secure a place for itself in the scientific scheme of things.

Clearly, empiricist epistemology was in crisis and that crisis infected any activity -like medicine or astronomy- that depended on an inference from the empirical success of a theory Abackwards@ to the truth of the postulates which generated the empirical success. Under such circumstances, astronomy would remain an activity of dubious credentials until either the crisis of empiricism was resolved or astronomy was properly axiomatized.

Powerful testimony to the magnitude of the crisis facing astronomy is provided in a famous passage from the that philosophical commentator of late antiquity, Simplicius. Paraphrasing an earlier argument from Geminus, he

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writes: Physics has the power to provide demonstrations concerning the size, shape and arrangement of [the heavenly] bodies. Astronomy is not prepared to say anything about this . . . It happens frequently that the astronomer and the physicist take up the same subject . . . But they do not proceed in the same way. The physicist must demonstrate every single one of his propositions by deriving it from the essence of bodies . . . The astronomer, on the other hand, is not equipped to contemplate causes, unable to tell us, for instance, what cause is responsible for the spherical shape of the Earth and the stars. . . . he fells obliged to posit certain hypothetical modes of being which are such that, once conceded, the phenomena are saved. . . .The knowledge of what is by nature at rest and what properties the things that move have is quite beyond the purview of the astronomer. He posits, hypothetically, that such and such bodies are immobile, certain others in motion, and then examines with what [additional] suppositions the celestial appearances agree.9

From a classical point of view, this characterization of astronomy is thoroughly damning: astronomers don=t give demonstrations, they don=t deal with causes, they posit hypothetical models and then merely check them against experience, unable to know whether what they have put in motion and what they have left at rest really are as their models describe. It would be bad enough if the astronomer had to depend on the physicist for causal explanations of the heavens. It is utterly devastating that the astronomer can but say that his models Asave the phenomena@ since nothing whatever follows from this about whether those models are true or false. The Medieval Interlude

With the fall of Rome, serious astronomy ceased to exist in what had been the Latin West (and almost everywhere else for that matter). By the tenth century, however, it was very seriously pursued in the Arab world, both in a variety of royal observatories and in many of the universities of Islam. Islamic astronomers had Arabic translations of the relevant texts of Aristotle, Euclid, and Ptolemy, among many others, and intellectuals had no difficulty quickly identifying the ambiguous attitudes of the Greeks with respect to the astronomy-physics nexus. Nor had they any trouble deciding who had won the standoff between Aristotle and Ptolemy over the scientific status of astronomy. With very few exceptions, Arabic astronomers accepted the argument that epicycles, eccentrics, and equants were implausible mechanisms, however successful they might be empirically. The Arab inference from that fact, however, was not to dismiss the

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pretensions of astronomy to scientific status. On the contrary, the remedy -as they saw it- was the development of a new astronomy that would avoid the awkward and ungainly mechanisms of the Almagest. Their would-be solution to the crisis of astronomy was, in effect, to deny that the crisis was epistemic, in the sense that the epistemic foundations of science were under threat. Rather, they insisted that the flaw must lie with the astronomy itself, whose models must be brought into line with Aristotle=s strictures on the nature of science. Accordingly, much of Islamic astronomy consisted in efforts to develop a theory of the heavens which, while rivaling Ptolemy=s empirical success at saving the phenomena, could do so while retaining those elements of Peripatetic physics that the Arabs held to be self-evident.

Specifically, as Pierre Duhem points out, Arab astronomers and physicists like Averros (1126-98) and Avempace (d. 1138) Atried to construct an astronomy

from which epicycles and eccentrics were both banished.@10 Generally, this

involved a revival of Aristotle=s theory of homocentric spheres, supplemented by new mechanisms added on to handle changes in apparent size and brightness. The Arabs had little patience with Ptolemy=s refinements, insisting (with

Averros) that Awe find nothing in the mathematical sciences that would lead us

to believe that eccentrics and epicycles exist.@11 While granting that Ptolemy=s astronomy excelled at saving the phenomena, they balked at the mechanisms he used to accomplish that task. Averros again: AThe epicycle and the eccentric are impossible . . . in our time astronomy is nonexistent; what we have is something that fits calculation but does not agree with what really is.@12 When he refers to

Afitting calculation,@ Averroes means, of course, that Ptolemy=s astronomy fits the observed motions of the heavenly bodies and enables us to predict their positions. But such empirical success as this counted for little among those who hankered after an a priori physics and astronomy. Despite strenuous efforts during two centuries and more, Arabic astronomers found no satisfactory replacement for Ptolemy=s unwanted geometric contrivances. But not even this failure was enough to force them to re-think their definition of what scientific knowledge amounted to.

By the time serious astronomy arrived again in Europe (the 13th century and beyond), much the same attitude prevailed as in Islam. Aristotle=s physics was accepted as gospel -by the philosophers if not the theologians. More important, the Aristotelian definition of science remained canonical. Judged against those yardsticks, the astronomy that the Scholastics inherited from the Greeks looked singularly unpromising. Thomas Aquinas (1225-74) spoke for the majority when he insisted, even while conceding that Ptolemaic astronomy fit the facts better than the system of homocentric spheres, that:

The assumptions of the astronomers [referring to the models of Ptolemy] are not necessarily true. Although these hypotheses appear to save the phenomena, one ought not affirm that they are true, for one might conceivably be able to explain the apparent

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motions of the stars in some other way of which men have not as yet sought.13

Astronomers like Ptolemy, according to Aquinas, sought to justify the belief in Aeccentrics and epicycles by the fact that we can save the sensible appearances of

the heavenly motions by this hypothesis.@ ABut,@ he goes on, Athis is not really a probative argument, since the apparent movements can, perhaps, be saved by means of some other hypothesis.@14

Aquinas=s refrain is by now familiar. What is wrong with Amerely@ saving the phenomena is that one does not thereby rule out the possibility that some alternative, but observationally equivalent, configuration represents the way things really are. Short of being able to rule out the other possibilities, one is left with mere opinion, and that amounts to neither knowledge nor science.

There were in the Middle Ages occasional dissenting voices to this consensus about the conventionality of mathematical astronomy. One astronomical voice in the wilderness was that of Bernard of Verdun. He argued that one can save the phenomena perfectly well with Ptolemaic epicycles and that, for all the talk of the possibility of other configurations, no one had actually been able to produce an alternative mechanism which worked anything like as well. Nothing else matters, says Bernard, except Athe truth of the conditional: If the epicycles and eccentrics did exist, the celestial motions and the other phenomena would occur just as they do now.@15 What Bernard does not explain, of course, is how to bridge the gap between a) our current ignorance of workable alternatives to Ptolemaic models and b) the claim that no such models exist, for the latter is what was required in order to establish the scientific status of quantitative astronomy. A science of Awhat ifs@ is no science at all. The Astronomical Revolution, Properly Speaking Almost everyone holds that the astronomical revolution was ushered in by the appearance of Nicholas Copernicus=s new heliocentric system. Many authors, from philosophers to poets, have waxed eloquently about the profound changes involved in Copernicus=s displacement of man from the center of the universe to an insignificant rock somewhere in the heavens. Profound though this change was, it is crucial to stress that Copernicus=s work did little or nothing to resolve the crisis about the scientific status of astronomy. Instead, it simply intensified that crisis. The Copernican heresy managed not only to reinforce the traditional philosopher=s conviction that mathematical astronomy was founded on

absurdities (Athe Earth moves@) but it also brought theologians, Catholic and Protestant alike, into the fray, who were mightily offended that astronomers like Copernicus would presume to correct Scripture. The theologians, like the philosophers, insisted that a mathematical theory of the heavens -whether geocentric or heliocentric- simply was not bona fide knowledge and ipso facto could not be genuine science so long as it depended on post hoc checking of its

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results. Besides providing additional theoretical reasons for worrying about the

status of astronomy, the system of Copernicus failed significantly to improve much on its Ptolemaic rival. Contrary to what some commentators have claimed, the Copernican system was not simpler than Ptolemy=s, requiring about the

same number of different epicyclic motions to explain the planet=s behavior as the latter did. (Yes, Copernicus did use epicycles.) Nor was it ontologically simpler in the sense of postulating fewer entities. For instance, Copernicus like Ptolemy had every planet carried about on a crystalline sphere.16 The astronomer and historian Owen Gingerich has argued persuasively that the best available Ptolemaic models of planetary movement yielded predictions that were of the same degree of (in)accuracy as those derived from Copernicus.17 The one place where Copernicus=s model was a significant improvement on Ptolemy=s, the motion of the Moon, provided no general argument for heliocentrism, since Copernicus=s Moon, as much as Ptolemy=s, was in a geocentric orbit. At the empirical level, therefore, there was likewise little basis for choosing between the two, especially until the invention of the telescope.

Copernicus published his principal astronomical work, De Revoutionibus, from his death bed in 1545. Containing a preface written by Andreas Osiander, its overseer on Copernicus=s behalf, the book created a stir even before its publication. Copernicus himself was an unabashed realist about the core premises of his new system. The Earth really moved, he maintained, and the Sun was the center of the Solar System. Historians of science have long reminded us of how counterintuitive both of these premises were. Is it possible that we could be on an Earth rotating on its axis and revolving about the Sun and still be completely unaware of that motion? The answer of Galileo (1564-1642), Copernicus=s self-appointed defender and advocate, was an unequivocal yes. Indeed, the whole point of what we still call the doctrine of Galilean relativity of motion is that the observable behavior of most things here on the Earth would be precisely the same whether it was absolutely at rest or in uniform motion.

That, of course, does not establish that the Earth is moving, only that it might be. Still worse, Galileo=s argument has the probably unintended consequence that it renders empirical observations of relative motion and rest impotent to decide between heliocentrism and geocentrism: a rock would fall more or less straight down whether it were dropping on a stationary or a moving Earth. (Once more, empiricism suffers at the hands of astronomers who have no tool other than experience for conducting their battles against the physics of Aristotle.)

That notwithstanding, Galileo maintained -as had Copernicus before him- that the heliocentric system is true and that astronomy must be considered a science capable of delivering veridical belief. But this was more bluster than argument. Neither Copernicus nor Galileo had definitive grounds for insisting that astronomy was a science in the classical sense (which both accepted as correct) of consisting of indubitable theories about heavenly motions.18 Nor, as

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Copernicus and Galileo themselves admitted in their more candid moments, did they have any proofs (as opposed to arguments from aesthetics or from mere plausibility) for the core theses of heliocentrism. Indeed, as noted before, Galileo=s most powerful and influential argument ostensibly in favor of Copernicanism is that there is nothing in observation or in a sound physics that refutes the heliocentric system.19 (This is rather like saying that we know that angels exist since there is nothing in our experience that refutes their existence.)

It follows that, if we would identify those who resolved the millennial crisis of astronomy, we must look elsewhere--principally to Kepler, to Descartes, and to Boyle- in order to grasp the complex of factors that led to the definitive enthronement of astronomy as the queen of the sciences . It was those theorists, not the philosophically challenged Copernicus nor the brash Galileo, who articulated and defended a notion of the nature of science that could finally secure an undisputed place for astronomy among the sciences and could, at the same time, resolve the epistemic crisis that had plagued astronomy since antiquity.

Let us begin with Johannes Kepler (1571-1630). Although best known among moderns for his three laws of planetary motion, and his meticulous derivation of the elliptical character of planetary orbits, Kepler=s interest to us lies elsewhere, especially in his lifelong project to work out the physics of the heavens. By that, I mean that he was concerned to figure out what forces caused the heavenly bodies to move as they did. Unwilling to accept the age-old idea that the planets moved in perfect circular paths (or paths compounded out of perfect circles), he broke much more sharply with his predecessors than Copernicus or Galileo had dared to do. Perhaps, he surmised, the planets don=t move in circles at all, and perhaps they are not carried along by moving crystalline spheres but by virtue of forces emanating from the Sun.

Kepler was suddenly blurring the classical boundaries between physics, astronomy, and cosmology, and was doing so quite deliberately. (The opening sentence of his most important book, THE EPITOME OF COPERNICAN ASTRONOMY, is the provocative salvo: AAstronomy is a part of physics.@20) In virtually all these investigations, his method was to postulate one or more hypotheses and then rigorously explore what the world would be like if those hypotheses were true. He knew perfectly well that this way of going on had been in bad epistemic repute for two millennia. That didn=t deter him because he saw, as few of his predecessors had, a way around the dilemmas that had been dogging empiricism for so long.

Basically, from the time of Ptolemy and even before, astronomers had seen the empirical checking of their hypotheses as simply a matter of comparing (some of) the consequences derived from their models against what we observe in the sky. An agreement between what our theories entail and what we have thus far observed was about all that pre-Keplerian empiricism could offer. Kepler insists that such a wholesale and indiscriminate empiricism is open to all the charges of bad logic and inconclusiveness that had traditionally been brought

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against it. But there is, he insists, another way to be a good empiricist. Kepler rightly recognizes that he is up against two quite distinct

challenges. One of them is that which arises from the presumed fact that, for any hypothesis which fits the facts, there will be (possibly many) others that fit the facts too. Let=s call this Kepler=s version of the problem of empirical equivalence of rival hypotheses. The second difficulty that any empiricist faces is the oft-repeated argument that since false hypotheses can have true consequences, the possession of true consequences by some hypothesis of interest does nothing to establish its truth. On this view of things, in order to show by experience or observation that an hypothesis is true, one would have to examine all its consequences and that is patently impossible for any typical scientific hypothesis. Let=s call this the problem of the partiality of testing.

The first explicit recognition that these were the two principal sources of epistemic unease about astronomy came a generation before Kepler. It was the Ptolemian, Christopher Clavius (1537-1612), who in 1581 explicitly formulated the hurdles that an empirical astronomy faced. It was, he insisted, the sceptics about the possibility of astronomical knowledge who must be confronted:

They concede that, on the assumption of eccentric and epicyclic spheres, it is possible to explain all the phenomena; but hold that it does not thence follow that the said spheres are to be found in nature: first, because it may be that all the appearances can be accounted for in some yet more convenient way as yet unknown to us; second, because it may well be that the phenomena can be explained by the said spheres even though they be purely fictitious and in no wise the true cause of those phenomena: just as a true conclusion may be inferred from a false premise.21

Clavius points out that this skeptical critique of astronomy has far-reaching implications, going well beyond astronomy itself. Indeed, he warns that the whole enterprise of natural science would be Adestroyed@ if one did not permit fallible inferences from observed effects to unobserved causes:

For whenever anyone inferred a certain cause from its observable effects, I might say to him what they [the skeptics about astronomy] say to us -that forsooth it may be possible to explain those effects by some cause as yet unknown to us. 22

If one is not justified in inferring from the phenomena that there are epicycles and eccentrics Ajust because a true conclusion may be inferred from a false premise, then the whole of Natural Philosophy will be destroyed.@23 Kepler has

precisely the same concerns, as we will see in a moment. I mention Clavius=s complaints first to stress that it was not only the Copernicans or heliocentrists who were reeling under the force of the skeptics= arguments about the illegitimacy of astronomical knowledge. Geocentrists as much as heliocentrists were bridling at the efforts of mainstream (but generally nonastronomical) thinkers to deny scientific status to the project of grounding one=s theories about the heavens on experience and observation.

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Kepler saw perfectly clearly that if the crisis of astronomy, and the attendant crisis of empiricism were to be ended, one would have to find plausible solutions to both the problem of empirical equivalence and that of partial testing. Otherwise, he feared, the astronomer would continue Ato be excluded from the

community of philosophers who inquire into the nature of things.@24 Although he grappled with these issues throughout his career, his most sustained effort to deal with them can be found in his Defense of Tycho against Ursus, written in 1600. He deals at greater length with the problem of empirical equivalence than with the problem of partial testing, though he has interesting things to say about each.

Concerning the problem of empirical equivalence, he says that people have jumped too readily to the conclusion that, if one hypothesis (or astronomical system) can save all the phenomena, then there must be others that can do so every bit as well. To the contrary, Kepler denies that two genuinely different astronomical hypotheses can ever really be such that they have all the same physical consequences. It is true, he concedes, that different hypotheses may yield the same predictions about observed locations of a planet on the celestial sphere. Yet line-of-sight observations do not exhaust the kind of evidence that an astronomer can collect. Consider the example of Venus. For the sake of argument, let=s assume that Ptolemy=s and Copernicus=s models each predict all the same coordinates for Venus as their rival does. Does this mean that we cannot use experience to discriminate between these theories? Certainly not. For instance, within the Ptolemaic system, where Venus is revolving around the Earth in an orbit between the Earth and the Sun, we would expect that, whenever Venus=s coordinates correspond with those of the Sun, Venus will pass between

us and the Sun, creating a dark spot on the Sun=s surface (technically known as a solar transit). By contrast, in the Copernican system, Venus will share the same celestial coordinates as the Sun whether it is passing on the far side of the Sun, relative to the Earth, or the near side. When the former circumstances obtain, Venus would simply be occluded rather than appearing as a dark spot on the Sun=s surface. Because of this difference, the astronomer in principle can use careful observations of Venusian transits to choose between the system of Ptolemy and Copernicus, because they are not observationally equivalent, even if they yield exactly the same celestial coordinates for a planet.

Kepler generalizes this sort of argument against observational equivalence, concluding that:

If a man assesses everything according to this method, I doubt indeed whether he will come across any hypothesis, whether simple or complex, which will not turn out to have a conclusion peculiar to it and separate and different from all others.25

People have been misled into imagining that there are many pairs of empirically indistinguishable hypotheses, Kepler thinks, because they have imagined that the sole astronomical task is that of predicting the place where a heavenly body will appear at a given time. Once we realize that there is more to a knowledge of the

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heavens than positional astronomy, we can recognize that most cases of seemingly equivalent hypotheses turn out to be empirically distinguishable. AEvery hypothesis whatsoever,@ he insists, Aif we examine it minutely, yields some consequence which is entirely its own and is not shared with any other hypothesis.@26

What about the second problem, that of the incomplete character of the testing process? Recall that the difficulty here arose from the universally acknowledged fact that false hypotheses may have some true consequences. Absent achieving the impossible (by investigating every possible consequence of an hypothesis), how can one be confident that a hypothesis that has so far stood up well to an investigation of its consequences will continue indefinitely to do so? What Kepler is up against here is nothing less than knowledge-certainty assimilation that had driven thought about these matters ever since Plato and Aristotle. Examining a finite number of the consequences of an hypothesis, even if we discover them all to be true, obviously cannot Aprove@ that the as-yet unexamined consequences will likewise be true.

Kepler=s strategic response is bold: he concedes the point, but denies its salience. Of course, he acknowledges, the ordinary, empirical checking of an hypothesis cannot prove it to be true as opposed to merely probable. But suppose, he continues, that the hypothesis can do some things out-of-the-ordinary? Suppose, for instance, that it not only explains the phenomena of the heavens that we have to hand but that it can also successfully predict presently unknown phenomena? Kepler stresses that what impressed him mightily about the Copernican system was Athe admirable agreement between his

[Copernicus=s] conceptions and all [the objects] which are visible in the sky; an agreement which not only enabled him to establish earlier motions going back to remote antiquity, but also to predict future [phenomena] . . . much more exactly than Ptolemy, Alfonso, and other astronomers.@27 Kepler seemed to believe that it was one thing for an hypothesis to explain why we already know about the world; it is quite another, and epistemically much more potent, if it can predict correctly things that we don=t now know.

If surprising predictions count for a great deal, so does a rather different trait of certain theories, viz., their ability to explain a very diverse range of different kinds of phenomena. While a false hypothesis may well be able to explain a few things successfully, we generally find that such hypotheses do not hold up well in the face of being applied to many and diverse facts. Kepler=s view

was that false hypotheses would quickly Abetray themselves@ by failing when they were applied to phenomena very different from those which suggested them in the first place.28

He cites one further trait of powerful hypotheses in order to quell the skeptic. Many hypotheses and systems, he says, can explain certain things about the heavens, while they leave other prominent features unexplained. (The hypothesis is not thereby falsified because its failure is not that it says false things

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about these phenomena; it simply says nothing at all. Kepler thinks that there is good reason to be suspicious of all such hypotheses that are silent where they should speak. He cites as an example the fact that Ptolemy=s theory postulates, but does not explain, why Mercury, Venus and the Sun all have the same period, one year. This is simply taken within Ptolemaic theory as a brute fact about the heavens. By contrast, the Copernican theory can offer an explanation for it, namely, that, being inferior planets moving about the Sun, Mercury, and Venus would appear from a moving Earth to require the same time as the Sun to complete a revolution. AThings,@ he says, Awhich arouse our astonishment in the case of other [astronomical systems] are given a reasonable explanation by Copernicus.@29 Kepler cites several other examples of celestial phenomena treated simply as brute facts by Ptolemy which receive a plausible causal explanation from Copernicus.

As should already be clear, what Kepler is struggling to do is to articulate a new version of empiricism, a new theory of experience. It begins with an explicit recognition (as the critics of empiricism had long argued) that simply knowing that the examined consequences of a hypothesis are correct is inconclusive. But, having granted that point, he does not draw their conclusion that scientific hypotheses are merely fairy tales. Rather, his approach is to stress that what is important is the kind of known consequences we are dealing with. If the examined consequences represent facts already known when the hypothesis was devised, then the hypothesis garners little support from them. If all the examined consequences involve the same sort of behavior, we likewise have no basis for credence. By contrast, if the hypothesis successfully predicts new kinds of things, if it explains a wide diversity of phenomena, and if moreover, it can explain facts that rival theories have to assume as unexplained, then Kepler thinks that the standard skeptical arguments no longer apply. When all these criteria are satisfied, thinks Kepler, we can be sure that the hypothesis we are dealing with is really true.

Clearly, Kepler=s is a highly nuanced empiricism. It replaces the

traditional empiricists= refrain about Asaving the phenomena@ by a recognition that some phenomena have much higher probative value than others. It responds to the sceptics= challenge that false hypotheses can have true consequences by insisting that false hypotheses cannot have true consequences of the particular sort that Kepler has described. Still, we have to ask ourselves: Is this enough to establish that such hypotheses are known with certainty and that they thus qualify as scientific in the canonical sense? Kepler obviously thought so as this revealing remark of his about Copernicus clearly reveals: ACopernicus thought his hypotheses were true . . . . And he did not merely think so but he proves that they are true.@30 Sad to say, Kepler=s explicit arguments won=t carry him and Copernicus this far. Even if it were so (which surely it is not) that no false theory ever predicted new phenomena successfully, explained a wide variety of phenomena, and explained many facts treated as unexplained primitives by its rivals, that would still be no more than a contingent empirical

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fact, insufficient to establish the certainty of any hypothesis that exhibits these virtues. To find the tools to leap that final hurdle, we must leave Kepler to one side and look to a succession of thinkers later in the 17th century who take the one step that Kepler was reticent to take: breaking with the demand for full certainty. The Invention of Moral Certainty Ren Descartes (1596-1650) tends to be known to modern audiences chiefly as a philosopher and inventor of the famous cogito argument. Among his contemporaries, however, Descartes was admired at least as much for his science as for his philosophy. He wrote books on geometry, optics, and astronomy. Naturally enough, it was in his scientific works that he considered at length the question of the status of astronomy as a form of scientific knowledge. Like Kepler and the whole astronomical tradition preceding them, Descartes acknowledged that the methods of astronomy do not permit the straightforward derivation of observable facts from one=s a priori first principles. Hence, an Aristotelian science of first principles is not possible in astronomy. Instead, one must, said Descartes, propound conjectures or postulates, which are then to be compared with what we see in the heavens around us. The crucial question, of course, was whether this feature of astronomy precluded its claims to possess infallible knowledge and, if so, whether that in turn preempted the possibility of a science of astronomy. Descartes=s response to this conundrum was different from

Kepler=s. Rather than insisting that the empirical checking of a theory can, under the right circumstances, produce certainty, he insists that the notion of certainty itself requires some teasing out.

Specifically, Descartes distinguishes between two types of certainty: what he calls Amoral certainty@ and the more traditional type associated with the truths of logic and mathematics. The first type, none other than our friend from chapter 2, will prove of paramount interest to our concerns. Basically, moral certainty is the sort of conviction we have about the most secure beliefs in our ordinary life. For instance, Descartes claims that although he never visited Rome, he is nonetheless Amorally certain@ that it exists, since so many ordinarily reliable witnesses have told him so. Likewise the beliefs that the sun warms the body or that water nourishes plants are morally certain. Of course, in principle, they could all be mistaken and thus these beliefs are not entitled to the same sort of certainty possessed by necessary, self-evident propositions, such as the truths of arithmetic or geometry. But, says Descartes, we have no reason whatever to doubt the existence of Rome and powerful reasons to accept its existence, just as we have no reason to doubt that the Sun provides warmth or that water nourishes. It is that situation -weighty arguments and evidence in favor of a belief and no independent, specific reasons to doubt it -that characterizes moral certainty. Put differently, we are morally certain of a belief when the only remaining doubts about it, if any, are those arising from a generalized scepticism, one which insists that possibly everything we believe is erroneous.

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What Descartes aims to show with this doctrine is that the disciplines of physics and astronomy contain theories that exhibit moral certainty; that feature, he will claim, entitles them to scientific status. But he faces a problem. It is one thing to argue for the moral certainty of those things that we see (or, as in the case of Rome, that others have seen); it seems to be quite another to argue that our beliefs about things that are both unseen and unseeable can exhibit this sort of surety. He explicitly acknowledges the problem he is up against. Speculating about the unseen world, he says, is akin to the conjectures one might make upon stumbling across a watch whose cover one could not open. One might theorize about its internal mechanisms, but how could that ever produce knowledge? He writes:

Just as an able watchmaker can make two watches that mark the hours in the same way even though their internal mechanisms have nothing in common, so is it possible that God possesses an infinity of diverse ways in which he might make all the things of this world appear exactly as they do, without it being possible for the human mind to discern which among those various means he actually chose to do things.31

We see here, of course, a version of the standard argument against the viability of inferring the unseen from the seen. Having acknowledged the problem, Descartes sets out to solve it, invoking a rather different metaphor. Imagine, he says, that we discover a letter written in code. We try to decipher the letter, substituting one letter for another, until -if we are lucky- we find that a certain substitution formula produces a message that is perfectly intelligible. In such circumstances, we immediately jump to the conclusion that our Adecoding@ is correct and that we have correctly identified the content of the message. But, of course, maybe our key was wrong, and we just happened on a decoding which, while producing intelligibility, does not get to the real message of the document. While improbable, says Descartes, that is nonetheless possible. But suppose, varying the example slightly, that the message is a very long one. Our confidence that we have the right key increases in direct proportion to the length of the message. The likelihood that a mistaken key would enable one to render intelligibly a very long message is so remote as to be inconsequential.

That, he says, is precisely the position of the physicist-astronomer who can use his hypotheses to explain things in the world as diverse as Athe heavens, the

magnet, light and fire.@ Under such circumstances, the conviction that our hypotheses have identified the Atrue causes of things@ becomes as strong as the conviction that a successful decoding of a lengthy message must be the right decoding.32 What justifies our confidence is not (as Aristotle would have liked) that we have deduced our hypotheses from some set of self-evident axioms. On the contrary, what makes us morally certain of the truth of our best scientific hypotheses are their impressive empirical credentials, that is, their ability to explain a great many, diverse things about the world.

It is useful to contrast this version of empiricism found in Descartes, with

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that exemplified by his contemporary, Francis Bacon (1561-1626). In his NOVUM ORGANUM, Bacon stressed the importance of the single, telling Acrucial

experiment,@ which would simultaneously confirm one hypothesis and refute its rival(s). Baconian empiricism shared the classical ideal of science as fully certain knowledge. Bacon=s self-appointed charge was to find ways to prove hypotheses beyond any possible doubt, using empirical methods and tests. Eliminative induction, that is, the systematic elimination via empirical refutation of all the possible rivals to a given hypothesis, was the last-ditch effort of the empirical tradition to cling onto classical certainty as the aim of scientific inquiry. Unlike Bacon, the advocates of moral certainty were not looking for the singleton Akiller@ experiment that would settle once and for all the truth of an hypothesis by vanquishing its rivals. On the contrary, they stressed that what produced moral certainty was a situation in which diverse and independent strands of argument and evidence pointed in the same direction. Moral certainty could never emerge from a single experiment or observation: it must be the product of a certain type of aggregation of confirming instances. Just as, in the law, one strand of evidence (say one witness who places the defendant at the scene of the crime) should b