Volume Two: Symposia and Invited Papers || Scientific Classics and Their Fates

Scientific Classics and Their FatesAuthor(s): Ernan McMullinSource: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association,Vol. 1994, Volume Two: Symposia and Invited Papers (1994), pp. 266-274Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/192936 .

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Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates Scientific Classics and Their Fates

Eman McMullin

University of Notre Dame

The term 'classic' runs a risk that all our terms of approbation face in these infla- tionary times, the risk, that is, of losing its ability to mark off singular works of human achievement. A "classic" today might be anything from a horse-race to a hair- style. But it was not always so. There is some debate about the origins of the term ("of the highest class"? "books for class use"?), but it long ago came to designate those works of Greek and Latin literature that for centuries shaped the education of young Western Europeans. These books were held up not only as models of literary accomplishment but also as sources of moral and social wisdom. Though they were composed in ages long past, their resonance was still felt in the present. And so the broader sense developed of a significant literary work of the past that still in one way or another speaks to us today. Thus Dante and Shakespeare and Racine could be added to the list that already contained Homer and Aeschylus and Horace.

1. Classics of science

Are there scientific "classics"? The term seems less often used in the context of scientific works. But why should it not apply to significant books in the history of science just as it does to chosen literary works? The problem is that major scientific works of the past do not seem to enjoy the sort of presence today that great works of literature do. They are rarely read, except by professional historians. They are in a real sense superseded, set aside, no longer consulted in the daily work of science. If someone today wants to explore the scientific issues that they once treated, that per- son would ordinarily consult the most up-to-date reference. Such works seem to fail the "resonance" test: people do not turn to them as they do to Homer or Dickens. (In that connection, however, I am tempted to recall that ancient definition of a classic as a book on everyone's shelves that hardly anyone reads!) Clearly, if we wish to speak of "classics" of science, then, we have to recognize that their relevance to the present is not of the sort that great works of literature might claim.

Inspired by Gadamer, Denis Sepper in our symposium distinguishes between two different usages of the term 'classic'. One is to designate, in a very general way, an outstanding example of a particular cultural movement or style. This he calls a stylistic-historiographical classic; its value to us is that it gives us an insight into the culture (style, mode of thinking) of which it is a representative. A subset of these qualifies, however, as classics in a more demanding sense: not only are they outstanding achievements in their own context, but they are part of a living tradition that endures

PSA 1994, Volume 2, pp. 266-274 Copyright ? 1995 by the Philosophy of Science Association

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to the present, hence the title: "classics of tradition". They "address us where we live"; they are "fecund even in the contemporary situation". Extending this distinction to science, and calling also on Kuhn, Sepper distinguishes between a general category of "paradigm-founding" works which can be classed as stylistic-historiographical classics even though they may be no more than "relics" in the contemporary scientific perspective, and classics of tradition which, like Euclid's Elements, can plausi- bly be held to retain a normative status in science even today.

Gadamer's terminology is cumbersome and the distinction he points to is far from sharp when applied to the history of science, as he himself would be the first to admit. There is a sense in which any scientific work of the past that would qualify in the first category would have some claim on the second category too. One does not have to subscribe to the old cumulative ideal of science, according to which progress consisted in laying brick on brick, with the old bricks retained intact, to note that major works of the past almost inevitably would find some echo in the science of today. The empirical regularities to which they point, for instance, might serve, under altered linguistic guise, as part of the evidential base of later theory. The difficulty of drawing the requisite distinction is brought out very well by the examples Sepper himself uses to illustrate it. As "relics" of only historical interest today he mentions Kepler's works on astronomy and Bacon's Novum Organon.. But Kepler's three "laws" of planetary motion and his insistence on a dynamical account of that motion did much to shape Newton's later achievement. And many of Bacon's prescriptions on method find an unmistakeable echo in our own century, in the work of Karl Popper, for example (Urbach 1982). The whole issue of retention, the extent to which later science typically incorporates earlier stages of inquiry has of course been the subject of sharp disagreement among philosophers of science in recent decades (see, for example, Feyerabend 1981 and McMullin 1984). In the circumstances, it seems wiser not to employ a distinction which relies for its force on the degree to which the classic in question remains "normative" at the present day.

The point of this discussion is not just a matter of word-usage. The quest is for those works of science that are most worthy of continuing study. The motives for such study would, of course, be various: historical (the effort to understand how a particular transition occurred, what the science involved conveyed about the world, what mode of evidence it appealed to); or philosophical (becoming clearer on how scientific inquiry has been carried on in the past, disentangling and evaluating specific arguments, testing contemporary theories of science against historical case-studies). Not every landmark in the past history of science is marked by a book, but when this is the case, such books offer an unrivaled resource for those who desire to understand the complex communal activity called "science". Let us agree, then to designate as "classics" those works that give us a special insight into the history and nature of science. Might one further require that they mark scientific revolutions, sharp shifts in research tradition, as Derek Gjertsen suggests (Gjertsen 1984)? This seems, at first sight, a plausible clarification. Ironically, however, none of the three works we have been studying in this symposium quite qualify under it. In the remainder of this brief commentary, I want to draw atten- tion to some of the points made in the course of the symposium, and to reflect on how strange the fates were of each of the three works that were discussed.

2. Aristotle's De Partibus Animalium

Aristotle scholars have long struggled with what Jonathan Barnes calls "The rob- lem of demonstration": "The method which Aristotle follows in his scientific and philosophical treatises and the method which he prescribes for scientific and philosophical activity in the Posterior Analytics seem not to coincide" (Barnes 1964). One possibility is that the model of demonstration described in Post. An. represents an early Platonic phase in Aristotle's thinking, and that the vast empirical research re- ported in his biological works belongs to a later and more mature phase (Jaeger 1934). Another is that the account of demonstration in Post. An. was put forward as

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an ideal of what knowledge itself should look like, not at all as an account of the procedures to be followed in actual inquiry (Randall 1960). Barnes' own solution is that Post. An. was intended as a model of how science should be taught, not how it should be acquired. William Wians, on the other hand, argues that dialectic (as outlined in the Topics), not demonstration, is Aristotle's chosen mode of teaching, that dialectic is a proper part of his theory of science and that this is the part exemplified by his biological treatises (Wians 1983). The great majority of recent commentators who have dealt with this issue (D.M. Balme, P. Moraux, G.E.R. Lloyd, are some others), though they disagree as to how to deal with the "problem of demonstration", agree at least that there is a problem.

But there have always been scholars who see no particular tension here. Indeed, this would have been the majority view among medieval commentators who assumed that even if explicit demonstrations are almost entirely lacking in Aristotle's works on natural philosophy, one could always, with a little ingenuity, convert his "proofs" to proper syllogistic form. A somewhat similar view is implicit in the work of several recent scholars (F. Solmsen and M. Grene, for example) who seem willing to allow many of the informal justifications given by Aristotle in his natural science to qualify as demonstration in a broad sense. However, James Lennox takes a bolder line in our symposium. Focusing on a single work, the De Partibus Animalium, he claims that PA I "was intentionally written to answer the question of how the Analytics model of science is to apply to Aristotle's paradigm natural substances, animals, and that PA H-IV carries out the program of PA I". Thus there is no "problem of demonstration" in the first place, for this work at least, and by extension, for the biological works generally.

The key to his argument is the notion of conditional (or hypothetical) necessity introduced in PA I, 1, where it is contrasted with the unqualified (or absolute) necessity which constitutes the ordinary meaning of the term for Aristotle. The notion is not a simple one. John Cooper sums up a detailed analysis of the relevant texts as follows:

An organ or feature of a living thing is formed by hypothetical necessity if, given the essence of the thing (specified in terms of capacities and functions) and given the nature of the materials available to constitute it, the organ or feature in question is a necessary means to its constitution.... Explanation by appeal to hypothetical necessity is not an alternative to explanation by reference to goals. It is a special case of the latter kind of explanation, the case where the independently given nature of the materials available for use in realizing the goal makes precisely one possible means, or some narrowly circumscribed set of possible means, to the end in question mandatory (Cooper 1987, 256; see also Balme 1987).

Aristotle remarks that this sort of necessity is appropriate to explanations of "things generated" (PAI, 642a 7), where final cause is primary. It is plausible, then, so Lennox argues, to suppose that Aristotle is here showing how to adapt the strong Post. An. notion of demonstration to the sciences of nature: simply weaken the notion of necessity required. One can then retain the general framework of Post. An., the emphasis on deduction from principles themselves seen, on the basis of experience, to hold with necessity. In this way we can formulate truths as mundane as that animal kidneys are necessarily fat-covered. The ways in which the parts of an animal serve the good of the whole organism, the functions each part plays in the larger whole, these are accessible to the student of living things in a way that the more abstract rela- tion of essence to property, on which the Post. An. had focused, is not.

I think that Lennox makes a good case. Does it dissolve the "problem of demonstration"? Not entirely. A number of reservations suggest themselves. First, his the- sis applies at best only to PA where conditional necessity is indeed important because of the stress on the relationship of part to function that runs through the book as a whole. But elsewhere in the works on natural science, even in the other biological

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works, this is much less evident. And even in PA, Aristotle stays for the most part at the descriptive level; passages like the one on the kidneys, where necessity is explicit- ly mentioned, are relatively rare. Post. An. specifies syllogistic form: there is not, I think, a single formal syllogism in all of PA. One can, of course, as Lennox does, convert various arguments there into syllogisms by supplying premises left implicit, adding the necessary quantifiers, and so forth. But this seems hardly enough to satisfy the Post. An. specifications, which would seem to require explicit syllogistic form.

Finally, a central concern in Post. An. is definition by genus and difference, and the kind of classification that this makes possible. Most of the examples given there are drawn from the living world. (See, for example, the application of the method of division to the highest genus, animal, in II, 13). One would have expected, then, a very different approach to the science of living things than is found in PA, where care- ful classification of this sort is notably lacking. Nor is it found in the other biological works either (Wians 1983). The linking of part and function by conditional necessity represents a significantly different approach. It seems plausible to suppose that when Aristotle began his researches into the living world he found that the necessary rela- tions of essence and property were in practice not accessible to epagoge (insight based on experience) in the way that the schema of science announced in Post. An. presumed them to be, so he turned to a much more manageable alternative instead.

In his concluding paragraph, Lennox himself notes that the paradigm of science underlying Post. An. is mathematics, and he asks to what extent a model of science drawn from mathematics (more specifically, from axiomatic geometry) can be applied to a world of teleologically-organized living systems. Though he leaves the question unan- swered, the answer to which the argument of his essay would seem to point is: hardly at all. And if this is the case, then his approach to the "problem of demonstration" cannot entirely satisfy. PA cannot be simply read, as he claims, as the application of the Post. An. model of science to the study of the living world. PA does propose an ingenious approach to at least one part of that study, but this approach, despite its reliance on a special kind of necessity, still marks a significant departure from the program of Post. An..

Did PA leave behind a thriving research program? The answer is: no, and this poses a new problem. In a recent essay, Lennox notes that although the period after Aristotle's death saw unprecedented developments in such "special sciences" as mechanics, astronomy, and optics, the study of zoology to which so large a part of his writings is devoted, vanishes from sight until the late Middle Ages (Lennox, 1994). Why? He rejects two possible hypotheses: that Aristotle's biological works themselves fell out of sight, and second, that there were not people of the right talents available to continue on the tradition. Galen, for example, refers many times to Aristotle's biological works, but shows no interest in carrying on the research pro- grams they embody. Lennox speculates that the sort of concern with the messy detail of the living world that the program called for would very likely not have been shared by many in the Hellenistic period, from what we know of the sensibilities of the time. It was not until the sixteenth and seventeenth centuries that others (like Cesalpino, Fabricius, Ray) were found who did share that concern and carry it further. Here, then, was a classic whose influence took two thousand years to show itself!

3. Galileo's Dialogue on Two Chief World Systems

There is nothing quite like Galileo's Dialogo in all the long history of Western science. It is constructed with consummate skill as an interlocking sequence of arguments directed to a single conclusion: the superiority of the Coperican over the Aristotelian and Ptolemaic world systems. It is intended for the general reading pub- lic, not for professionals, and hence for the most part avoids technicality without losing its logical force. It is a paragon of literary elegance, one of the seminal works that helped to form the Tuscan ' canon" of the Italian language. Its publication set off a theological firestorm whose embers still occasionally flare.

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In his Galileo and the Art of Reasoning (1980), Maurice Finocchiaro provided an extended analysis of the logic of the Dialogo, lauding it as a treasurehouse of argumentative strategies. Some of his conclusions he summarizes for us in his symposium paper. I will be concerned here only with what he has to say about the role played by the Dialogo in the "Galileo affair". Galileo was condemned by the Church authori- ties in 1633 because the Dialogo was held to violate the norms laid down by the Holy Office in 1616. This is the nub of the Galileo affair. But did Galileo in fact violate the restrictions laid on him? Finocchiaro believes that a "plausible claim" can be made that he did not. The Dialogo in his estimation is an "impartial" work, presenting the arguments on each side of the Copernican debate in a fair manner: "Rather than supporting or defending the earth's motion [outlawed by the mandate laid on him in 1616], Galileo decided to simply discuss the arguments.... He must have felt that there could be nothing wrong with stating, analyzing, and evaluating the arguments on both sides. This was a plausible, realistic and viable program for operating within the restrictions".

Though I can agree with much of the supporting argument Finocchiaro brings, I think that this conclusion is mistaken, and it is on a matter of great moment in any as- sessment of the trial of 1633. He is not merely claiming to show how Galileo could have believed himself to be operating within the norms laid on him, thus explaining why he pursued a strategy that in its outcome was so disastrous for him. He is claiming that a good case can be made for saying that in fact the Dialogo did not contra- vene those norms.

It would require much more space than I have at my disposal to deal adequately with the intricacies of this much controverted issue. Let me go back very briefly to the events of 1615-16, when the stage was set for what was to come. It was at that point that the Holy Office, the Roman Congregation concerned with matters of doctrine, made the fatal error. What happened later in 1633, when the author of the Dialogo was brought to trial, though far more dramatic than the events of the earlier time, was the sort of consequence that could have been predicted.

In 1615, as the storm clouds gathered in Rome around the Copernican doctrine, Galileo composed one of the most interesting theological documents of the century, the Letter to the Grand Duchess Christina. He asked: what is the Christian to do when there is an apparent conflict between a Biblical passage, taken in its literal sense, and some finding of natural science? The standard answer which had been formulated by St. Augustine a thousand years before (call it the A principle for short) was that the literal reading of the Bible should be maintained unless the scientific claim could be demonstrated, in which case an alternative reading of the Biblical passage should be sought. Galileo repeats this hermeneutic principle, with apparent approval. But he also proposes and argues effectively for a very different one, linking it with a bon mot attributed to Cardinal Baronio: the purpose of the Scriptures is to tell us how to go to heaven, not how the heavens go. The Bible, he concludes, carries no weight in matters of natural philosophy since its books were not written for that purpose. Thus, a conflict between the two cannot in principle arise, and objections to particular scientific doctrines cannot call on Scripture in their support (call this the B principle).

Finocchiaro makes two claims, each of which I would dispute. One is that B pro- vides the rationale for A, thus Galileo in proposing B is showing, among other things, why the traditional principle holds. The second is that an immediate corollary of B, that a natural philosopher should be free to propose a doctrine at apparent odds with Scripture even where he cannot demonstrate the doctrine, can be justified by the same arguments as would justify A. Thus anyone who accepts A ought to accept this corollary, the one that Galileo so badly needed to defend his procedure in the Dialogo.

First, B does not provide a rationale for A. If B were correct, the natural philosopher would not be obliged to provide a demonstration to sustain his side when apparent conflict threatens. Second, the rationale given for A by Augustine and later the-

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ologians in no way would warrant B or the corollary drawn from it. B is a much broader principle, and in the context of the debates then raging between Catholics and Reformers, a far more dangerous-sounding one. What follows from all this is relatively simple: if Galileo is ruled by A, the principle that his theological critics will hold him to, he has to produce a demonstration of the earth's motion in the Dialogo in order to gain a hearing. If, on the other hand, he goes by B, a plausible case for the earth's motion will suffice, but his critics are not in the least likely to accept the principle that would permit this. What made matters even more complicated for Galileo were the restrictions that Bellarmine communicated to him in the aftermath of the 1616 decree, i.e. explicit orders not to "hold or defend" the Coperican doctrine. These instructions implicitly contradicted the B principle, so that Galileo could not call on it to justify his planned defense of the Coperican system. But if he were to follow the A principle, he would have to produce a demonstration of the earth's motion. He was not at all sure, I suspect, that he had a demonstration: good grounds, yes, but a strict demonstration, hardly. Worse: he had been forbidden even to defend the condemned view, so how could he produce a demonstration, even if he had one? It seemed a hopeless tangle.

When the new Pope, his friend Urban VIII, encouraged him in 1624 to proceed with his plans to write about the Coperican controversy, he thought he saw a way through the tangle. He was authorized by Urban (so it would seem) to treat Copernicanism as an "hypothesis". But therein lay an ambiguity that proved disastrous in its consequences. What Urban would most likely have meant was that the Copernican formalism could be used merely for calculational purposes, the standard sense of the term 'hypothesis' in the context of mathematical astronomy for many centuries before. Whereas what Galileo evidently took away from his meetings with the Pope was that he could offer evidence in support of the truth of the Copemican world system, so long as he did not lay claim to demonstrate it. (This, in effect would be our modern understanding of the term, 'hypothesis'.) Sly attempt to circumvent the restrictions laid on him in 1616? Wishful thinking? Genuine misunderstanding? It is impossible to say. But it was his own sense of what constituted hypothesis, not Urban's, that guided the construction of the Dialogo (McMullin, 1978b).

Now, to Finocchiaro's principal claim: that the Dialogo did not violate the prescription against holding or defending the suspect doctrine. The importance of this issue is, of course, that it was on the opposite claim that the Holy Office case against Galileo primarily rested. Finocchiaro argues that the prescription would have been violated only if 1) the evidence for Copericanism were presented as completely conclusive, or 2) that the case made on its behalf failed to be impartial. I would question both parts of this. Nothing was said in 1616 about impartiality. The prescription for- bade Galileo to defend Copericanism. But an impartial (that is, fair) presentation of the evidence could just as easily count as a defense as a partial one would. (I am not so sure, furthermore, that Galileo did make his case in an impartial way. Finocchiaro himself brings out how effectively loaded against the Aristotelian side was the rhetoric of sarcasm and insult that Galileo employed.) Nor was the prescription laid on Galileo in 1616 limited to forbidding claims to demonstrate the Copernican system. "Defending" is much weaker than that: to defend, it would be sufficient to present the evidence on one side as much stronger than on the other.

And (as Finocchiaro himself allows) Galileo certainly did do that. Over and over again, the arguments he advances in the Dialogo are said to favor the Copernican side, to "strengthen the Copemican hypothesis until it might seem that this must triumph ab- solutely" (Preface). In concluding the Dialogo, just before the argument he inserts in deference to Urban, he says: "In the conversations of these four days we have, then, strong evidences in favor of the Coperican system, among which three have been shown to be very convincing [the apparently irregular motions of the planets, the paths of the sunspots, the ebb and flow of the tides]". Speaking of the tidal argument in particular, Finocchiaro himself remarks that Galileo 'was unwilling or unable to seriously

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consider any alternative explanations." In short, then, readers of the Dialogo would have been in no doubt that the author aimed to defend the merits of the Coperican over the Aristotelian alternative. And that, of itself, would have been sufficient to warrant the accusation that he had violated the prescriptions laid on him. His accusers at the trial made a point of this. ( I am bracketing some famously disputed questions concern- ing the actual procedures followed at the trial.) What had been banned in 1616, they pointed out, was not just the claim to demonstrate the truth of the Copemican doctrine but even the attempt to show it to be well-supported, i.e. probable. This would be "a serious error since there is no way an opinion declared and defined to be contrary to Holy Scripture may be probable" (Trial sentence, Finocchiaro 1989,289.)

Neither Galileo nor his oppontents, Aristotelians all in matters of logic, knew how to deal with the awkward intermediate category of likelihood or probability, the category that, so obviously to our eyes, the Copernican debate called for. Nor could they know that the issue of realism (the reality in this case of the earth's motion dependent upon the explanatory force of the Copernican theories ) would still be debated more then three centuries later. Given the conflicting constraints under which the Dialogo labored, it could never have achieved the primary goal its author had set for it. But it could and did play a significant part in a paradigm-shift more profound, perhaps, than any other in the history of science.

4. Newton's Opticks

Half a century later, Newton was still struggling with the issue of the admissibility of probable reasoning in science proper. Alan Shapiro in his masterly reconstructions of Newton's abundant optical writings has shown how this concern shaped the Opticks of 1704, (Shapiro 1989, 1993). By Newton's day, a number of natural philosophers, like Boyle and Huygens, had argued for the acceptability of hypothesis as a proper part of science, and had proposed criteria that such hypothesis would have to satisfy (McMullin 1990). But Newton's disposition was quite otherwise. In his first lectures at Cambridge (1668-69), he set out to treat optics as a part of mathematics: "Although colors may belong to physics, the science of them may be considered mathematical" (Shapiro 1993, 25). And when, shortly after, Hooke described Newton's explanation of the spectrum produced by the refraction of sunlight by a prism as a "hypothesis", Newton was incensed. As far as he was concerned, it was a straightforward "deduction from the phenomena", with no shadow of the hypothetical or the probable about it. Others could, if they wished, call on "mechanical hypothesis" to give an account of how the colors themselves are caused, but in Newton's view this was "foreign to the purpose" of science. A few years later, he relented, under pressure to make his mathematical treatment of light "more intelligible", and developed a highly speculative theory of an ethereal medium compounded of a variety of active "spirits' whose action might explain not only the phenomena of the spectrum but also, perhaps, those of electrical at- traction and even muscular movement (McMullin, 1990, 68-71).

When in 1687, with the Principia completed, Newton started to assemble his earlier writings on light and color with a view to a second major treatise, he determined to return to his original idea of a science of light that would have nothing of the hypothetical about it. It would be broadly deductive in logical form, deploying as the start- ing-point of deduction the various experiments on light he had so brilliantly devised in the 1670's, in addition to some new work on thick plates that offered new insight. The opening lines of the Opticks sixteen years later recall this goal but give no hint of the difficulties to which it led: "My design in this book is not to explain the properties of light by hypotheses, but to propose and prove them by reasoning and experiments". Shapiro has followed the working and reworking of the materials that occu- pied so much of Newton's time in the intervening years. No matter how hard he tried to avoid them, causal hypotheses linking the evident periodicity of the color phenomena to some sort of vibrations in the medium or elsewhere seemed to be the only way in which the observational results could be drawn into unity.

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Finally, he hit on the notion of "fits of easy reflection and easy transmission", an abstract set of mathematically-describable dispositions, that would (he hoped) pre- scind from the awkward question of what mechanisms were causally responsible. But it was not enough. After the introduction of the "fits", the Opticks contains a long section in which dozens of new observations of assorted color phenomena are set forth. The text then breaks off abruptly ( "I was suddenly interrupted....") leaving this mass of observational material unanalyzed. It is difficult to avoid the conclusion that he was unable to carry further his aim of constructing a non-hypothetical science of directly describable properties. Instead, he ends the work with the famous series of "Queries" in which all sorts of causal hypotheses are introduced without embarrass- ment, but where his conviction that they are merely heuristic devices intended only to aid in the formulation of mathematical propositions that make no physical or causal statement, remains unchanged.

What Sepper calls the "texture" of the Opticks is determined in large part, then, by this effort to separate the task of the natural philosopher into two, one mathematical and conclusive, the other "physical" and speculative and of heuristic value only. This strategy had worked reasonably well for him in the Principia, though critics like Leibniz would have none of it (McMullin 1978, chap. 4). But it led him astray in dealing with the far greater variety of optical phenomena. A more tolerant approach to hypothesis and probable reasoning would have led to an Opticks of a very different texture, one whose legacy to later generations of scientists might have been a more effective one.

For, it must be said, the Opticks was not a classic in the fullest sense. It did not initiate a successful research program nor a fundamental paradigm shift. Despite the extraordinary brilliance of the experimental designs it unveiled, despite the wealth of invention displayed in the suggestive mechanisms of the Queries, it gave mixed sig- nals to those who attempted to follow its lead. The theory of fits found few supporters in the century that followed, and had little influence on the later development of wave theory. Supporters of Huygens' wave theory were opposed by Newtonians who defended the corpuscular emission theory implicit in the light-ray model employed in the formalism of the Opticks. Following a Newtonian lead, some tried unsuccessfully to use the concept of force to describe the interaction between light-ray and medium.

In Newton's defense it could be said that in the light of what we now know, specu- lation about the causal mechanisms underlying the phenomena of light was indeed pre- mature. The irreducibly dual wave and particle aspects of light could not have been harmonized in the language of Newton's mechanics; a different mechanics would be needed, one that would not be tied to the inductivist ordinary-language presuppositions of the Principia. It could be argued, then, that nothing that conceivably lay within Newton's reach could have established a successful paradigm for optics at that time.

But the main negative effect of the Opticks lay elsewhere, in the attitude of distrust towards explanatory hypothesis that it encouraged. Such hypotheses were not entirely proscribed, but they were to be regarded as heuristic devices, dispensable aids to be laid aside once the desired mathematical description of the properties of the phenomena under investigation was reached. The "physical" side of natural philosophy, with its appeal to hypothetical underlying causal mechanisms, was to be subordinated to the "mathematical". The philosopher, Thomas Reid, constructed a tightly empiricist philosophy of science around these and similar Newtonian dicta, one that would still carry weight a century later when J.S. Mill was writing his System of Logic. But in the later eighteenth century, the reliance on fluids and ethers of all sorts in the theories of heat, electricity, and especially optics, posed a severe challenge for any philosophy of science that would call on this side of the divided Newtonian heritage. Perhaps the best that can be said for the Opticks in this regard is that it displays, to a quite striking degree, an ambivalence that philosophy of science has not yet quite overcome.

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Documents

Volume Two: Symposia and Invited Papers || Scientific Classics and Their Fates