Logic, Thought, and Action 14

Chapter 14

AGENTS AND AGENCY INBRANCHING SPACE-TIMES∗

Nuel BelnapPittsburgh University

Abstract Branching time puts an indeterminist causal structure on instantaneousbut world-wide “super-events” called moments. This theory has “actionat a distance” as an inevitable presupposition. In contrast, branchingspace-times puts an indeterminist causal order on tiny little point events.The benefit of branching space-times theory is that it can represent lo-cal indeterminist events with only local outcomes. The “seeing to itthat” or “stit” theory of agency developed in Belnap, Perloff and Xu,2001 employs branching time as a substructure, and thus has the fol-lowing shortcoming: It is inevitably committed to an account of action-outcomes that makes them instantaneously world-wide. This essay askshow the stit theory of agency can be adapted to branching space-timesin such a way that action-outcomes are local.

The aim of this essay is to make some suggestions for the beginnings ofa theory of agents and agency in branching space-times. The thought isto combine the ideas of agency as developed by Belnap, Perloff and Xu,2001 against the relatively simple background of branching time withthe richer notions of mere indeterminism as structured in the theoryof branching space-times. My plan is to say a little about agency inbranching time and a little about branching space-times, and then askhow the two can be brought together.

1. Stit theoryIn this section I offer some brief and general remarks in connection

with “stit theory” as described at length in Belnap, Perloff and Xu, 2001

∗An earlier version of this essay appeared in the Journal of Sun Yatsen University (SocialScience edition), vol. 43, 2003, pp. 147–166.

D. Vanderveken (ed.), Logic, Thought & Action, 291–313.©c Springer. Printed in The Netherlands.2005

292 LOGIC, THOUGHT AND ACTION

(henceforth FF).1 In the course of these remarks, I will occasionally referto the following designedly boring example.

Peter will drive his car to work one day hence. [1]

What are the chief elements of research strategy for FF? The centralaim is to use formal theory to help a little in understanding humanagency as it works itself out amid the (we think) indeterministic causalstructures of our world. We call it “stit theory.” It takes its name fromthe prominent use of a non-truth-functional connective:

α sees to it that Q,

which we abbreviate with

[α stit: Q]

In connection with stit, the analysis moves in several directions, as indi-cated in the remainder of this section.

1.1 Stit thesesIn an effort to tie the formal grammar of stit to natural language, FF

offers a series of so-called “theses.” The theses, which I paraphrase fromthe appendix to FF, are these:

Agentiveness of stit thesis. English is ambiguous, and even theEnglish “α sees to it that Q” is ambiguous, but [α stit: Q], incontrast, is designed to be unambiguous: It invariably attributesagency to α.

Stit complement thesis. As a matter of grammar, Q can be anysentence whatsoever, including cases in which [α stit: Q] turns outtrivially false, e.g. [α stit: the sun rises every morning].

Stit paraphrase thesis. An English sentence is an agentive if andonly if it can be usefully paraphrased as a stit sentence, includingthe case in which Q is paraphrased as [α stit: Q], e.g. [Peter stit:Peter drives his car to work].

Imperative content thesis. The content of every English imperativeis agentive, hence equivalent to a stit sentence; e.g., the declarativecontent of “Peter, drive your car to work” is [Peter stit: Peter driveshis car to work].

1See also publications listed there, and in particular, Horty, 2001.

Agents and Agency in Branching Space-Times 293

Restricted complement thesis. For many English constructions,e.g. “α promises that ,” it is illuminating to restrict their com-plements to future-tensed agentives, hence to stit sentences; e.g.,Peter promises that one day hence [Peter stit: Peter will drive hiscar to work].

Stit normal form thesis. When worried about difficult questions ofagency, it is generally useful to paraphrase each agentive as a stitsentence.

Together these theses say that in thinking about the philosophy of action,it is helpful to use [α stit: Q] as a normal form for expressing that anagent does something — something that natural languages express in abewildering variety of fashions. As applied to [1], this gives

One day hence:[Peter stit: Peter drives his car to work]. [2]

Note that we have used “One day hence:” as a connective in order toexpress the desired tense structure with complete explicitness.2 This isimportant in being clear about indeterminism.

1.2 Stit theory and indeterminismStit theory assumes indeterminism. Why? Stit theory is equally pre-

humanist and pre-scientific. We take it as a fact that our agentive do-ings together with non-agentive happenings are enmeshed in a commoncausal order. The sense of “causal order” here should not be pickedup from a determinist presupposition. The appropriate causal order isindeterministic. Nor does this involve “laws.” Why should it? Bare in-determinism is what is wanted: There are initial events in our world forwhich more than one future is possible. It is this simple concept of inde-terminism that permits progress on the theory of agency. Thousands ofpages written by philosophers about agency make (or seem to make) thecontrary assumption of strict determinism. Hume and Kant are famousin modern philosophy for what has come to be called “compatibilism”,the doctrine that agency is compatible with strict and absolute and per-fect determinism. On the other hand there are also hundreds of pages(I’m making up these numbers) assuming some form of indeterministic“free will”; one has to think only of William James and his rejection ofthe idea of a “block universe.” Rarely, however, is an indeterministicview of agency combined with a desire to let the enterprise be guided bya desire for mathematically rigorous theories. The aim of our research is

2We have dropped the “will” of [1] as logically redundant.


to make an indeterminist account of agency “intelligible” (Kane, 1998).We try to pursue this aim by means of a simple and rigorous theory.

1.3 The metaphysics of agencyThe simplest representation of indeterminism is this: A treelike struc-

ture that opens toward the future. That is, FF, following Prior andThomason, represents indeterminism by means of a partial order inwhich there is no backward branching. We call the elements of thetree moments. A moment is an entire slice of history, so to speak, fromone edge of the universe to the other. A moment is not a “time”; it is aconcrete “super-event” (Thomson, 1977) caught up in the causal order,with its unique past and its future of possibilities. A maximal chain ofmoments is called a history, and represents one particular fully-detailedway in which our world might go or might have gone. Do not in yourmind identify a single history with a “world”; instead, it is the entire tree-like assemblage that is to be identified with Our World. Philosophicallogicians generally use the term branching time for such a structure, andwe shall do the same, even though “branching histories” would be lessmisleading. The right side of Figure 1 gives a picture.

In FF we argue at length against the pernicious doctrine that one ofthe many possible histories through a moment has a special status as“actual” or “real.” FF shows that our understanding of assertion, pre-diction, promising, and the like is much improved if all histories througha moment are treated on a par, with none singled out.

One reaches the “theory of agents and choices in branching time” byadding two elements to the bare theory of branching time. (1) Agentis a set whose members are considered to be agents capable of makingchoices and so acting. (2) Choice is a function that is defined on allmoments m and agents α, and which delivers a partition of all the histo-ries through m. The partition represents the choices open to agent α atmoment m. We may call each member of the partition a choice that isavailable to α at m, or a possible choice for α at m. There is one serioustheoretical constraint on Choice: If two histories h1 and h2 in the treedo not split from one another until after a certain moment m, then nochoice available to α at m can distinguish h1 and h2. This we call “nochoice between undivided histories.” As we often say, you cannot maketomorrow’s choices today.

When there are multiple agents under consideration, one naturally(?) assumes that the simultaneous choices of two agents α1 and α2 areradically independent; technically, we assume that every choice by α1 isconsistent with every choice by α2 when their choices are simultaneous.


That’s it. As you can see, our “metaphysical” theory of the causalstructure of agency is about as minimal as can be.

1.4 The semantics of indeterminismSimple as it is, the branching-time structure does not explain itself,

especially not to philosophers who assume determinism as an unques-tioned “fact.” One has to come to terms with what prediction, andmore generally the use of the future tense and other kinds of statements“about” the future, mean in connection with the tree representation ofindeterminism. This is a matter not of metaphysics, but of semantics.The problem is how to understand certain linguistic structures underthe assumption that they are used in an indeterministic situation. Thesolution to that problem, including agency as a special case, lies in com-bining the Prior-Thomason semantics for branching time, in which truthis relativized to both moments and histories, with the Kaplan semanticsfor indexical expressions.

The Prior-Thomason-Kaplan semantics makes the truth of e.g. [1]relative not only to the moment of utterance, but in addition relativeto a moment-of-evaluation parameter needed for understanding tenseconstructions that take one away from the moment of utterance, and,crucially, relative also to a history-of-evaluation parameter that repre-sents a way in which the future can unfold. The semantics is entirelytwo-valued, since relative to each moment of utterance, moment of eval-uation, and history of evaluation, there is a definite truth value givento the example sentence. If a sentence is true [false] relative to everyhistory through a given moment of evaluation, the sentence is said tobe settled true [false[[ ] at that moment. Picture [1] or [2] as asserted bysomeone on Monday in a situation such that whether or not Peter willdrive to work one day hence is still an open question. The semantics,as explained in FF (and in more detail in Belnap, 2002a), makes it pos-sible to say precisely: When asserted on Monday, [1] is neither settledtrue nor settled false. However, it is a settled truth on Monday that,no matter what happens, one of the following holds: Either on Tuesdayit will be settled true that [1] was true at the moment of assertion (onMonday), or on Tuesday it will be settled false that [1] was true at themoment of assertion (on Monday). With slightly more brevity (but stillwith inevitable risk of confusion): The assertion of [1] is not settled oneway or the other on Monday; but no matter what happens, on Tuesdayit will be definitely settled whether or not the assertion was true at themoment of assertion.


This confusing passage involves “double time references,” which aresorted out both in FF and in Belnap, 2002a. It may also be called“bivalence later” (Copley, 2000). The subtle part is that [1] is evaluatedwith respect to a Monday moment m0 that is the moment of assertion,and with respect to each history h that passes through not m0, butinstead through some Tuesday moment m1 at which we are evaluatingwhether or not the assertion was true at the moment of assertion onMonday. In this way, the theory provides a framework for normativelyevaluating assertions about the future in terms of what we may call“fidelity”: We say that an assertion is vindicated or impugned at somelater moment depending on whether it is settled true or settled false atthat later moment that the assertion was true at the moment of assertion.

1.5 The semantics of agencyOn the basis of the Prior-Thomason semantics that relativizes truth

to moment-histories pairs, we suggest two principal semantic analysesof stit. These analyses depend essentially on an indeterminist view ofagency. Both share the idea that action begins in choice, and that there isno choice and therefore no action without the availability of incompatiblechoices. We work out the semantics of stit in two ways. Both involve atransition from an unsettled situation to settledness due to the choice ofan agent.

The achievement stit postulates that the relevant choice occurredat some temporal remove in the past of its settled outcome.

The deliberative stit works through a concept of action based onthe choice being in the immediate past of its settled outcome.

In either case, the semantics says that [α stit: Q] is true at a certainmoment m with respect to a certain history h provided (1) a prior (per-haps immediately prior) choice of α guaranteed the truth of Q, and (2)the choice was a real choice among incompatible alternatives. Put tech-nically, the truth conditions for [α stit: Q] at a pair m/h when taken asthe deliberative stit come to this:

1-1 Definition. (Stit) Positive condition. Q is true at m with respectto every history h1 that belongs to the same possible choice for α at mas does h . (This is the part that says that α’s choice at m guaranteesthe truth of Q.) Negative condition. Q is not settled true at m: Thereis some history h2 to which m belongs such that Q is false at m withrespect to h2. (This says that α really does have a choice at m that is


relevant to the truth of Q.)

The semantic analysis that FF gives to the achievement stit is lesssatisfying. I nevertheless give a brief version here. First of all, one isrequired to add a “time” parameter (FF says Instant) to the underlyingtree structure, with the assumption that all histories are temporally iso-morphic. This amounts to the presumption that one can make sense outof comparing the “times” of incompatible moments. The achievementstit relies on this, letting the “positive condition” be, roughly, that [αstit: Q] is true at m/h iff Q is settled true at every moment that (1) isco-temporal with m and (2) lies on a history that is in the same choicefor α at m as h .

1.6 StrategiesStit theory allows a sophisticated but simple account of strategies,

conceived of as a pattern of prescribed choices for future action. Inthis part of the theory, the principle of “no choice between undividedhistories” in modeling agency is critical. The principle, you will recall,denies that our world allows us to choose today between histories thatonly divide tomorrow. In a nutshell: To choose a strategy is not tochoose the later choices that define it.

It follows that we must carefully distinguish the availability of a strat-egy from having a strategy. In other words, the stit theory of strategiesmakes it imperative that one distinguish “acting in accord with a strat-egy” from “acting on a strategy.” This is a special case of the following:The fundamental notions of stit and strategies may serve as a kind offoundation for intentional notions. If you start that way with the in-determinist causal structure of agency, you easily see how difficult itis to sort out the intentional notions in relation to causal notions inany way that keeps them sorted out, and you are more likely to avoidself-deception in appraising your theories.

Almost breathlessly I have highlighted five parts of the stit theoryof agency: the stit theses, how stit theory presupposes indeterminism,the metaphysics (causal structure) of agency as we see it, the difficultsemantics of indeterminism in general, the truth conditions for stit, and,finally, the stit theory of strategies. It is this apparatus that I suggestshould inform our discussion of how agency fits into branching space-times, and I shall presuppose it when I turn to this matter in §3. First,however, I drop agency in order to survey some main ideas of branchingspace-times.


2. Branching space-timesThere is much less written on branching space-times (BST, as I shall

say), and it is less accessible, in part because most essays on this topicwere written with physical examples such as quantum mechanics inmind.3 Perhaps the easiest approach to BST starts from a Newtonianpicture of the world of events.

Newtonian universe. Non-relativistic and deterministic: World= Line. The Newtonian universe has, as I see it, two features thatare so fundamental that they can be described without advanced mathe-matics, using only the characteristics of the causal-ordering relation andits relata. First, the items that are related by the before-after causalorder are momentary (= instantaneous) super-events: Laplace’s demonneeds total world-wide information concerning what is going on at time t .Second, the causal order is strictly linear: For any two momentary eventsm1 and m2, either m1 lies in the causal past of m2, or vice versa: m2 liesin the causal past of m1. It is the linearity of the causal order that an-swers to determinism, and it is the global conception of the items fallinginto a causal order that answers to non-relativistic “action at a distance”:An adjustment in positions or momenta here-now can immediately callfor an adjustment over there in the furthest galaxy. The picture of thecausal order in a Newtonian universe is therefore a simple line, with eachpoint representing a world-wide simultaneity slice, all Nature at a cer-tain time t . In such a universe there are not histories (plural), but onlya single History, so that we may say that World = History; this makesthe Newtonian universe deterministic. Furthermore, and independently,the relata of the causal ordering are momentary super-events; this makethe Newtonian universe non-relativistic. With this in mind, we may saythat on the Newtonian view, World = Line as on the left side of Figure1.

Branching space-times is to be both indeterministic and relativistic.Since the Newtonian universe is both deterministic and non-relativistic,it takes two independent moves to make the transition from the Newto-nian universe to branching space-times.

Branching time universe. Non-relativistic but indeterminis-tic: World = Many Lines. In the first move away from Newton

3 BST theory in the form deriving from Belnap, 1992 is discussed in the following places:Szabo, Belnap, 1996, Rakic, 1997, Belnap, 1999, Placek, 2000a, Placek, 2000b, Belnap, 2002e,´Mueller, 2002, Placek, 2002b and Belnap, 2002b. Belnap, 2002d gives an overall view of bothstit theory and BST theory.


Figure 1. Newtonian universe and Branching-time universe

we keep the relata of the causal order as momentary super-events, sothat we remain non-relativistic. In order to represent indeterminism,however, we abandon linearity in favor of a treelike order. The resultof this first transition from the Newtonian universe, when taken alone,is exactly what we have already discussed under the rubric “branchingtime.” In branching time there is indeed a single world, but instead ofthe equation World = Line, the world of branching time involves manyline-like histories, i.e., many possibilities: Branching time is indeter-ministic. Since, however, we have kept the causal relata as momentarysuper-events, branching time remains non-relativistic: Splitting betweenhistories in branching time has to be a world-wide matter of “action ata distance” since the consequences of the split are felt instantaneouslythroughout the farthest reaches of space. We may therefore say that ac-cording to branching time, World = Many Lines that split at world-widemomentary super-events as on the right side of Figure 1.

Einstein-Minkowski universe. Relativistic but deterministic:World = Space-time. The other move away from Newton is thatmade by Einstein in principle, and more explicitly by Minkowski. Toobtain the Einstein-Minkowski universe from that of Newton, we keepdeterminism from the Newtonian universe; there is no trace of alternativepossible futures. The change is rather that now the terms of the causalrelation are no longer simultaneity slices (momentary super-events) thatstretch throughout the universe. Instead, the fundamental causal relataare local events, events that are limited in both time-like and space-like dimensions. When fully idealized, the causal relata are point eventsin space-time. This, to my mind, is the heart and soul of Einstein-Minkowski relativity. The move to local events is made necessary byEinstein’s argument that there is simply no objective meaning for asimultaneity slice running from one edge of the universe to the other.There is no “action at a distance”: Adjustments at e1 influence onlyevents e2 in “the forward light cone” of e1, or, as will say, in the causal


future of e1. I wish to urge that not only fancy Einstein physics, buteven our ordinary experience (when uncorrupted by uncritical adherenceto Newton or mechanical addiction to clocks and watches) shows us thatevents are not strung out one after the other. Take the event of our beinghere now at e1. Indeed some events lie in our causal future, so that thereare causal chains from e1 to them, and others lie in our causal past, sothat the causal chains run from them to e1. But once we take localevents as the relata of the causal order, there is a third category, alwaysintuitive, and now scientifically respectable, since we have learned tobe suspicious of the idea of (immediate) action at a distance. In thisthird category are local events e2 that neither lie ahead of e1 nor dothey lie behind e1 in the causal order. Letting < be the causal orderrelation, I am speaking of a pair of point events e1 and e2 such thatneither e2<e1 nor e1<e2, Instead, e1 and e2 have a space-like relationto each other. Neither later nor earlier (nor frozen into simultaneity bya mythical world-spanning clock), they are “over there” with respect toeach other. Einstein makes us painfully aware that space-like relatednessis non-transitive, which is precisely the bar to the objective reality ofmomentary super-events. Events in their causal relation are not reallyordered like a line. Our modern reverence for various parts of Newtonianphysics and our related love of clock time delude us.

Since the Einstein-Minkowski relativistic picture is just as determin-istic as the Newtonian picture, there are no histories (plural), but onlyHistory, so that we have the determinist equation World = History. Thedifference from the Newtonian picture is with respect to an independentfeature: A causally ordered historical course of events can no longer beconceived as a linear order of momentary super-events. Instead, a his-tory is a relativistic space-time that consists in a manifold of point eventsbound together by a Minkowski-style causal ordering that allows thatsome pairs of point events are space-like related. Therefore, if we makethe single transition from the Newtonian universe to that of Einstein-Minkowski, the result is that World = Space-time as on the left side ofFigure 2.

Branching space-times: relativistic and indeterministic: World= Many Space-times. BST now arises by suggesting that thecausal structure of our world involves both indeterminism and relativis-tic space-times; we are therefore to combine two independent transitionsfrom the Newtonian universe. We can already make a certain amountof capital out of that suggestion. For indeterminism, we shall expectnot World = History but World = Many Histories. For relativistic con-siderations, we shall expect that each history is not a line, but instead


Figure 2. Einstein-Minkowski universe and Branching-space-times universe

a space-time of point events in something like the Einstein-Minkowskisense. Therefore, in BST we should expect that World = Many Space-times as on the right side of Figure 2. Furthermore, just as histories inbranching time (each of which is like a line) split at a world-wide mo-mentary super-event, so in branching space-times we should expect thathistories (each of which is like a space-time) should split at one or morepoint events. Technically, we represent our world as Our World, whichis a set of (possible) point events, and we let e range over Our World.

We need, however, more information about how the various histories(= space-times) fit together. What is analogous here to the indeter-ministic way in which branching-time structured individual Newtonianline-like histories into a tree? A crucial desideratum is this: The the-ory should preserve our instinct that such indeterminism as there is inour world can be a local matter, a chance event here-now that has noeffect on the immediate future of astronomically distant regions of theuniverse. It needs zero training in mathematics to see that the the-ory of how BST histories fit together into a single world will be morecomplicated than the branching-time theory that arranged many linesinto a single tree. I mention four key points underlying the theory of“branching space-times.”


2.1 Consistency in BSTHistories are closely related to the ideas of possibility and consistency.

A guiding idea here is that what allows two events to share a history,and therefore to be consistent, is that at least one event lies in theircommon future. As long as there is a standpoint in Our World fromwhich one could truly say “both of these events have happened,” evenif the two events are not themselves arranged one after the other, onemay be confident that the two events can live together in single history.Peter’s (possible) driving to work in one village and Paul’s (possible)staying home in another village are consistent just in case there is some(possible) standpoint at which someone could truly say, using the pasttense, that both events came to pass. Let us also turn this around: If twoevents are inconsistent, then no event can have both of them in its past.For example, although Peter’s choice to drive to work is altogether local,and although we cannot picture Our World as a tree, nevertheless, thereis no standpoint anywhere in Our World that has in its past both of theinconsistent events represented by Peter’s having driven to work and hishaving stayed home. These inconsistent possibilities can and must lieahead of the point at which he makes the choice, but they cannot bothlie behind anything whatsoever. A single maximal consistent set of pointevents will be a space-time; we call it a history, we let Hist be the set ofall histories in Our World, and we let h range over Hist.

2.2 Choice points local, not globalThere have to be choice points, definite local events at which two

histories split into radically inconsistent portions. It is presumably nottrue and we must not assume that when splitting occurs, it occurs insome magical world-wide way. When Peter is given the choice to driveor to stay home on a certain occasion, that occasion is confined in spaceas well as in time. His little bit of free will is local, not global. And thesame might be true when the choice is only metaphorical, a matter of arandom outcome of some natural event such as, perhaps, the decay ofa radium atom in Paris. It might be that the decay is a strictly localmatter, neither influencing nor influenced by contemporary happeningsin, say, Manhattan. Whenever there is indeterminism, whether of choiceor of chance, a good theory must give meaning to the difficult idea thatthe indeterminism is local, not worldwide.

Important in BST theory are the same ideas of undividedness and ofsplitting that are so important for the FF theory of agency. For laterreference we indicate some pieces of notation that are useful in speaking


of these matters.

2-1 Definition. ((H(( (e), h1≡e h2, Πe〈h〉, Πe, and h1⊥e h2)) H(e) is theset of all histories to which e belongs. h1≡e h2 means that histories h1

and h2 are undivided at e (they split at some point event that is prop-erly later than e ). Πe〈h 〉= {h1: h ≡e h1} is the immediate possibility ate to which h belongs, and Πe is the set {Πe〈h 〉: h ∈ H(e)} of all imme-diate possibilities at e . h1⊥e h2 means that h1 and h2 split exactly at e .

2.3 Prior choice principleWhen I put the third point in everyday language, it sounds so obvious

that you will yawn. And yet as far as I know a thoroughly controlledstatement has never been made apart from the present theory of branch-ing space-times. The BST postulate for locating choice points may beput informally in the following way: Whenever we find ourselves as partof some contingent event instead of in a history that is an alternative tothe occurrence of that event, we may always look to the past for a choicethat serves as a locus of the splitting. This is an axiom of BST theory;I call it the prior choice principle.

Example. Suppose that on a certain Tuesday Peter is in his of-fice, having driven to work that morning. Think of this as a particularconcrete event, with a definite causal past, and let the event be contin-gent, i.e., an event that has not been fated from all eternity. Take anyhistory in which that event fails to occur, perhaps a history in whichPeter spends the whole of Tuesday high in a tree in the Amazon jungle.The theory guarantees that if you look in the causal past of the givenconcrete Peter-in-office event (the one that we are supposing occurred),you will find a definite choice point at which things could have goneeither in the direction of keeping the Peter-in-office event possible, or inthe direction of keeping the Peter-in-tree history possible.4 You do notneed to look in the future, and you also do not need to look far awayat events going on “over there.” The point is that examination of thecausal past of the Peter-in-office event suffices. (The theory does notpresume to say if the choice point belonged to Peter or to a lion or to abit of natural randomness or, perhaps, to some combination.)

4The theory will not let you exchange “event” and “history” here; precision of statement isessential.


2.4 Funny business?The fourth point records a recognition of an important way in which

the theory should not be strengthened. The following is a principle thatan unwary philosopher might easily be inclined to endorse.

Tempting principle (no funny business). If two choice pointsare related in a space-like way, so that the second is “over there” withrespect to the first, then their respective choices are entirely independentof each other. This I call “no funny business.” Aristotle gives us asimple example of a causal story without funny business. He tells ushow two market-goers meet at the market, as an “accidental” result oftheir choices that morning, made separately in far-away villages. Theirindividual choices, we all suppose, are bound to be totally uncorrelated,that is, independent, and that is what the tempting principle of no funnybusiness says must be so.

The theory of branching histories, however, resists this temptation.And it does not do so a priori. It does so because Our World seems, asa matter of fact, to contain violations of the tempting principle. Withreference once more to Einstein, quantum mechanics seems to tell us thatin fact it is possible for two utterly random choice points to be space-like related, with no hint of a line of causal connection between them,and nevertheless fail to be independent. This is “funny business.” InBelnap, 2002e it is shown that this form of no-funny-business is provablyequivalent to the following form: Every situation that is cause-like withrespect to a certain outcome event lies in the causal past of that event.5

There is more. Reichenbach has taught us that whenever we find thelong arm of coincidence stretching across space, it is in our nature tolook for a common cause. Funny business precisely happens when thereis objective coincidence — which is to say, a failure of independence— across space, without a common cause. Belnap, 2002c shows thattwo forms of “no common cause funny business” are equivalent to thetwo accounts of “funny business” of Belnap, 2002e. Since modern-dayphysics apparently says that funny business happens, it is good that thetheory of branching histories has room for funny business — and indeedhas the virtue of permitting us to offer a stable (albeit conjectural)account as to the difference between (1) mere indeterminism but withno funny business and (2) indeterminism with funny business.

5If you replace “every” by “some,” you have a different statement, one that is serious businessinstead of funny business, and one that is provably guaranteed by the aforementioned priorchoice principle.


2.5 Summary of BSTThese necessarily too-brief points allude to a theory of branching his-

tories that gives a satisfying account of how physical indeterminism canbe local instead of global. It gives us an account of how choices andoutcomes of natural random processes can affect only what lies in theircausal future, touching neither their past nor the vast region of space-like related events. But it does so in such a way as to allow plenty ofroom for individually random space-like-related processes to be, as thephysicists say, “entangled.” Or, in the phrase I just used, the theory ofbranching histories helps us to come to terms with funny business.

The result is that the theory of branching histories, in addition tohelping us clarify ideas of action and agency, provides low-key sugges-tions for articulating some of the strangest phenomena uncovered bycontemporary physicists. It does this by avoiding careless or fuzzy orsloppy formulations. It does this by insisting on a careful and rigorousaccount of what it is for indeterminism to be not immodestly global, butmodestly local.

3. Agents and agency in branching space-timesSince branching space-times is more realistic and more sophisticated

than branching time as a representation of the indeterminist causalstructure of our world, it is natural that one should consider agentsand agency in branching space-times. The matter is hardly understoodat all, which is precisely why it should be investigated. I offer sometentative thoughts on how some of the fundamental ideas might go.

Agents in branching space-times. First off, what shall we dowith the concept of an agent? In FF the concept is represented by aset, Agent, and to be an agent, α, is merely to be a member of this set.Nothing is said about the “real internal constitution” of an agent beyondmembership in the set Agent. Instead, the FF postulates describe agentsonly insofar as agents make choices. FF presumes that every agent hasa choice at every moment (for technical convenience counting vacuouschoices as choices), where the construction Choiceα

m(h) represents thechoice that agent α makes at moment m on history h . Since the choicesof a set of agents Γ at the same moment are to be taken as simultaneous,it is natural that FF postulates such a set of choices to be independent:No combination of individual choices for the members of Γ is impossible.Behind these postulates lie the FF idea that, since moments are takento be world-wide in extent, many agents can “occupy” a single moment.Recall §2: The chief difference between non-relativistic branching time


and relativistic branching space-times lies in what “the causal order re-lation” relates. In the former case, moments are super-events taken tobe “spatially” rich enough to be “occupied” by more than one agent. Inthe latter case, point events would seem to be so small as to admit the“presence” of at most one agent. For this reason it seems reasonable tobegin by representing an agent as a set of point events, the set of pointevents that the agent may be thought of as occupying in the course ofhis or her life.6 Continuing to use Agent as the set of agents, we maytherefore begin with the following.

3-1 Tentative postulate. (The agent as a set of point events) Everyagent is a set of point events: ∀α[α ∈ Agent→ α ⊆ Our World.7 Whene ∈ α, we may say that the point event e is part of the agent α, or thatα is located at or occupies e .8

Given that we are going to represent an agent α as a set of pointevents, what constraints make sense? In the beginning it seems best,since easiest, to think of the life of an agent in a particular history as aportion of a “world line.”

3-2 Tentative postulate. (Agents and world lines) The portion ofthe life of an agent in a particular history is a chain of point events:∀α∀h[(α ∈ Agent& h ∈ Hist)→ α ∩ h is a chain in Our World.9

Since point events in α can belong to many histories, and although theportion of α in each is a chain, it is easy to see (and is provable) that theentire set α will look like a tree, which accurately represents that thereare alternative future possibilities for the life of α.10 I hope it is needlessto say that I am claiming for this postulate only that its simplicity makesit a good beginning; it may well turn out to be better to represent anagent in a single history as a cloud of point events rather than as achain. The thought is that nothing more that Tentative postulate 3-2 is

6In contrast, in branching time it would make no sense at all to represent an agent as a setof moments!7In this study I am not after a “fundamental” or “exclusive” ontology of agents. Here arepresentation counts as useful if its structure leads us to helpful theory.8None of these phrases is entirely happy, but given that in any event we are not trying tosay what agents “really are,” perhaps it doesn’t matter.9A chain in Our World is a subset of Our World such that each pair of distinct membersare comparable by the causal ordering relation. A chain may be empty.10I counsel you to have no patience with those who make fun of this picture by (falsely)describing it as saying that it is a possibility for tomorrow that Peter both be in his officeand also stay at home.


desirable for “first purposes.” For example, it may be that the track ofan agent in any one history is dense or continuous; but at this stage ofinquiry I doubt that it matters one way or the other.

Let us think of the joint representation of two or more agents. Sincepoint events are so small, it seems plausible that one should never havemore than one agent located at a point event. Perhaps, then, we shouldenter the following.

3-3 Tentative postulate. (No agent overlap) Agents never overlap:∀α1∀α2[(α1, α2 ∈ Agent& α1 �=�� α2)→α1 ∩α2 = ∅].

That requirement may, however, be too strong; it might be better tosay only that no nonvacuous point event (no point event with more thanone possibility in its immediate future) can be shared by two agents. Theweaker restriction would not entirely forbid that the “world lines” of twoagents intersect.

Choices and stits in branching space-times. The foregoing ten-tative postulates suggest (but certainly do not demand) that the choicesopen to an agent α at a point event e are exactly the same as the im-mediate possibilities at e in the sense of BST theory. In other words,there may well be no need for imposing a Choice function in additionto the possibilities definable from the BST causal ordering alone. Clar-ity will be heightened, however, if we introduce the Choice notation foragents as a separate primitive governing agents in branching space-times.

3-4 Notation. (Choice) Assuming that agent α occupies a point evente belonging to a history h , Choiceα

e (h) is a new primitive to be readas “the choice available α at e to which h belongs.” Also Choiceα

e , de-fined when e ∈ α, as {Choiceα

e (h): h ∈ H(e)}, is to be read as “the setof choices available to α at e ,” and Choice is defined as that functiondefined for every agent/point-event pair α and e such that e ∈ α thatdelivers the set of choices available to α at e .

Introducing Choiceαe (h) as a primitive leaves open the possibility that

some interesting ideas turn out to need Choice as a separate concept.Since Πe (Definition 2-1) is the BST notation for the set of immediatepossibilities at e , defined in terms of undividedness at e , the followingpostulate is to the effect that when a point event e is a part of agent α inhistory h , then there is no difference between the choice available at e forα that contains h , and the immediate possibility at e to which h belongs.


3-5 Tentative postulate. (Choice(( αe (h) and Πe〈h〉) Let an agent α

occupy a certain point event e . The undividedness-at-e relation betweenhistories of Our World determines what is immediately possible at e .We postulate that there is no difference between what is immediatelypossible at e and what α can choose at e : ∀e∀α∀h[e ∈ (α ∩ h) →Choiceα

e (h) = Πe〈h 〉].

This postulate is not without content: It says that the choices con-cerning the immediate future that are available to an agent at a certainpoint event e are very finely articulated indeed. According to Tentativepostulate 3-5, there is no finer articulation that Nature (or other agents)can impose beyond the choosing powers of the agent himself for his im-mediate future. On the other hand, and with equal realism, choices andhappenings in the near vicinity of e can and doubtless will limit thepowers of the agent concerning his non-immediate future. Joint agency,for example, will not concern what is immediately possible, as it doesin FF; instead, the outcomes of joint agency will come to pass at somespatio-temporal remove from the choices of the various agents involved.Such is of the very essence of the relativistic flavor of agents in branchingspace-times.

Already we can spell out a notion of agent responsibility for immedi-ate outcomes along the lines of FF. Once we adapt to BST by lettingtruth be relative to pairs e/h (with e ∈ h ) rather than pairs m/h , thedefinition of the deliberative version of stit seems forced (compare Defi-nition 1-1): [α stit: Q] is true at e with respect to h iff e ∈ α and

3-6 Definition. (Stit in BST) Positive condition. Q is true at e withrespect to every history h1 ∈ Choiceα

e (h). (This is the part that saysthat α’s choice at e guarantees the truth of Q.) Negative condition. Qis not settled true at e : There is some history h2 ∈ H(e) such that Q isfalse at e with respect to h2. (This says that α really does have a choiceat e that is relevant to the truth of Q.)

Agent causation in branching space-times. There should bea rich possibility for notions of agent causation of outcomes adaptedfrom Belnap, 2002b. Let O be a “scattered outcome event,” definedas a consistent set of outcome chains (chains that are nonempty andlower bounded). A “cause-like locus” for O is a point event e whoseoccurrence is consistent with the occurrence of O and which is suchthat what happens there makes a difference to the occurrence of O:∃h [h ∈ (H(e) ∩ H〈O〉) &h⊥eHO]. Funny business happens when some


cause-like locus for O is not in the causal past of O, and we know (or, viastandard quantum mechanical puzzles going back to Einstein, we thinkwe know) that funny business happens in the natural world. Whetherchoices of agents can exhibit funny business is presumably not somethingto be decided by fiat; nevertheless, it seems much the best to begin ex-ploring agency in the absence of funny business, and so we have to beable to say what that means. I am far from sure of the best thing tomean by “no agentive funny business,” but the following, though proba-bly inadequate, seems like a reasonable first suggestion to be pondered.

3-7 Definition. (Agentive funny business) A pair consisting of an out-come event O and a point event e counts as agentive funny business iffe is part of some agent α and e is a cause-like locus for O and e is notin the past of any part of O.

3-8 Tentative postulate. (No agentive-funny-business pairs) Thereare no pairs O and e that count as agentive funny business.

It seems that Tentative postulate 3-8 suffices to rule out at least somecases of superluminal agent causation, and is in the vicinity of saying thatthe choices of agents make a difference only to the future. It is doubtful,however, that the postulate is strong enough to rule out all forms ofagentive funny business; more analysis is needed than we have so farprovided. A further strengthening might consider a joint choice initialinvolving an arbitrarily complex (but consistent) set of choice points,each of which belongs to some agent. It would seem that the wholecomplex should be independent of any joint choice initial, no matterhow complex, and no matter if agentive or natural or mixed, as long asthe purely agentive joint choice is space-like related to the other complex.

Transitions are important in BST. We can be interested in an “ef-fect” transition I �→O, which is an ordered pair of an initial event Iand a scattered outcome event O, with every member of the initial inthe causal past of some member of the outcome. Such a transition is“contingent” if there is a dropping off of histories in the course of thetransition, and in such a case we may with profit ask for a causal accountof the matter. The answer is to be given in terms the set cc(I �→O) ofall of the causae causantes or “originating causes” of I �→O. For thisconcept we first define the set cl(I �→O) of “cause-like loci” for I �→Oas {e :∃h[I ⊆h & h⊥eH〈O〉]}. These point events are exactly where “theaction happens” in keeping O possible at the expense of ruling out al-tenative possibilities. Then cc(I �→O) = {e �→ (H(e) ∩ H〈O〉) : e ∈cl(I �→ O)}; these are the transitions that “do the work.”


In simple cases, which are the only ones that are so far well under-stood, we shall find all of cl(I �→O) in the past of O, with no funnybusiness. Often cc(I �→O) will involve a mixture of agentive choices andnon-agentive happenings; for example, the winning may have been partlycaused by how the coin came up (e.g. heads came up), and partly bywhich bet was chosen (e.g. the bet was on heads). We know that givenno funny business, the set of cc(I �→O) form a set of “inns” conditionsfor I �→O: Each member is an insufficient but non-redundant part of anecessary and sufficient condition (whence the acronymn “inns”) for theoccurrence of the outcome O given the occurrence of the initial I. Non-IIredundancy means in particular that the complete causal story for I �→Ocannot leave out any member of cc(I �→O). Furthermore, we can parti-tion cl(I�→O) into those cause-like loci that belong to α1, α2, etc., andthose that do not belong to any agent. In this way we may expect finecontrol over our causal descriptions. For example, if cl(I�→O)⊆α1 ∪α2,we can say that the two agents α1 and α2 were between them entirelyresponsible for the transition I �→O.

Message-sending in branching space-times. Such joint respon-sibility is not of course the same as joint action. Our primitives, sincebeing purely causal, do not permit discussion of factors such as “jointintention” that may be thought to underlie joint action. We can, how-ever, say a little bit about the causal aspect of the communication thatseems to be required for joint action. Look at Figure 3.

Figure 3. Sending a message in branching space-times

What is represented causally is that α2 sends a message to α1. Mes-sage-sending is agentive, and so there is a choice, in this case a choiceby α2 whether to send (say) a red message or a blue message. There isindeterminism in the life of each agent, but only the sender is agentivein the matter; α1 is entirely passive. If the two are to “collaborate,” onewould expect that a message sent by α2 to α1 would (not dictate but)limit the choices open to α1 at a subsequent choice point, depending on


red vs. blue; and of course further collaboration would involve agencyon the part of α1 in choosing a message to α2. If an “intentional” storyinvolving the minds of α1 and α2 cannot be told as an enrichment of thebare causal tale told by Figure 3, then it is hard to see how it could beuseful.

It is striking that Tentative postulate 3-8 says nothing about non-agentive funny business such as occurs, or seems to occur, in EPR-likefunny business. I should think that it is essential to think through thecombination of “no agentive funny business” with “some EPR-like non-agentive funny business.” Even physicists and philosophers of physicsseem to distinguish between the independence constraints put on thechance outcomes of measurements on the one hand and on the deliberatesettings-up of measurements. There is, however, too little talk aboutwhy there should be a difference, and its exact nature. It is plausiblethat the structure of agency in BST can help in thinking about this.

Choice and state in branching space-times. One last thought.In developing an evaluative theory of choices by an agent α in branchingtime, Horty, 2001 introduced the idea of a “state,” which Horty definedin effect by freezing the simultaneous choices of all agents except forthe target agent α. Since in branching time the simultaneous choices ofdistinct agents are always independent (every combination is possible),he was in a position to consider the familiar two-dimensional diagram inwhich rows represent the choices available to α, columns represent thevarious possible “states” (independent of the choices of the agent), andthe intersections are labeled with evaluative information. In transportingthis idea to BST, one must consider from the beginning that space-likerelatedness in BST, unlike simultaneity in branching time, is definitelynot a transitive relation, any more than is the same relation in Minkowskispace-time. It follows that a choice by an agent α can well be space-like related to each of two choices by a second agent, those two choicesbeing causally ordered (successive), so that although each is independent(by space-like relatedness) of the choices of α, they are not independentof each other. Worse (if that is the right word), branching space-timesallows that choice points for other agents, even though space-like relatedto the given choices of α, are outright inconsistent with each other. Oneis therefore going to have to put in extra work — and work that shouldbe carried out — in order to locate a satisfying account of “state” thatis eligible to do the same work as Horty’s account in branching time.It seems to me that in working through this nest of considerations, onewould naturally be led to a useful theory of “games in branching spacetimes” that would take with utmost seriousness the causal structure of


the players and the plays in a fashion that sharply separates (as vonNeumann’s theory does not) causal and epistemic considerations. Itis one thing for two choices to be independent of each other, and adifferent thing for there to be certain relations of (say) ignorance. Somuch is clear. It is not of course self-evident that the difference makesa difference. If it does not, however, that should be the result of seriousreflection, not an unconsidered presupposition.

4. SummaryI have tried to sketch some of the main ideas of stit theory, drawing

on FF, and I have given a kind of overview of branching-times theoryas discussed in the publications listed in note 3. Then I have raisedbut definitely not answered some questions concerning how to use thesematerials in order to fashion a theory of agents and their choices inbranching space-times. I have suggested with all too much brevity somepossible lines of investigation.

ReferencesBelnap N. (1992). “Branching Space-Time”. Synthese 385–434.— (1999). “Concrete Transitions.” In Actions, norms, values: Discus-

sions with Georg Henrik von Wright. G. Meggle, Ed. Berlin: Walterde Gruyter. 227–236. A “postprint” (2002) may be obtained fromhttp://philsci-archive.pitt.edu

— (2002a). Double Time References: Speech-Act Reports as Modalitiesin an Indeterminist Setting. In Wolter et al, 37–58. A preprint of thisessay may be obtained from http://www.pitt.edu/∼belnap

— (2002b). A Theory of Causation: Causae causantes (Originating Causes)as inus Conditions in Branching Space-Times. A preprint of this essaymay be obtained from http://philsci.archive.pitt.edu.

— (2002c). No–Common-Cause EPR-Like Funny Business in BranchingSpace-Times. Forthcoming in Philosophical studies. A preprint of thisessay may be obtained from http://philsci-archive.pitt.edu

— (2002d). Branching Histories Approach to Indeterminism and FreeWill. This essay may be obtained from http://philsci-archive.pitt.edu

— (2002e). EPR-like ‘Funny Business’ in the Theory of Branching Space-Times. In Placek and Butterfield 2002, 293–315. A preprint of thisessay may be obtained from http://philsci-archive.pitt.edu

Belnap N., Perloff M. and Xu M. Facing the Future: Agents and Choicesin our Indeterminist World. Oxford: Oxford University Press. 2000.


Copley B. (2000). “Conceptualizing the Futurate and the Future”. InMIT working papers in philosophy and linguistics: The linguistics /philosophy interface, Bhatt R., Hawley P., Kackl M. and Maitra I.eds. Cambridge MA.: MIT Press. 45–72.

Horty J.F. (2001). Agency and Deontic Logic. Oxford: Oxford UniversityPress.

Kane R. (1998). The Significance of Free Will. Oxford: Oxford UniversityPress.

Muller T. (2002). Branching Space-Time, Modal Logic and the Coun-¨terfactual Conditional. In Placek and Butterfield, 273–291.

Placek T. (2000a)“Stochastic Outcomes in Branching Space-Time: Anal-ysis of Bell’s Theorem”. British journal for the philosophy of science,51 :445–475.

— (2000). Is Nature Deterministic?. Krakow: Jagiellonian University´Press.

— (2002b). Partial Indeterminism is Enough: a Branching Analysis ofBell-Type Inequalities. In Placek and Butterfield 2002, 317–342.

Placek T. and Butterfield J. (20020). Non-Locality and Modality. Dor-drecht: Kluwer Academic Publishers.

Rakic N. (1997). Common-Sense Time and Special Relativity. Ph. D.´thesis. University of Amsterdam.

Szabo L. and Belnap N. (1996). Branching Space-Time Analysis of theGHZ Theorem. Foundations of physics, 26, 8:989-1002.

Thomson J. (1977). Acts and Other Events. Ithaca: Cornell UniversityPress.

Wolter F., Wansing H., de Rijke M. and Zakharyaschev M. “Advancesin Modal Logic”. World Scientific Co. Pte. Ltd, 2002:3. Singapore.

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Logic, Thought, and Action 14