salmon 1994 pos causality without counterfactuals

8/8/2019 Salmon 1994 POS Causality Without Counterfactuals

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Philosophy of Science Association

Causality without CounterfactualsAuthor(s): Wesley C. SalmonSource: Philosophy of Science, Vol. 61, No. 2 (Jun., 1994), pp. 297-312Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/188214

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CAUSALITY WITHOUT COUNTERFACTUALS*WESLEY C. SALMONtt

Philosophy DepartmentUniversity of PittsburghThis paper presents a drastically revised version of the theory of causality,basedon analysesof causal processes and causal interactions,advocatedin Salmon(1984). Relying heavily on modified versions of proposals by P. Dowe, thisarticle answers penetrating objections by Dowe and P. Kitcher to the earliertheory. It shows how the new theory circumvents a host of difficulties that havebeen raised in the literature. The result is, I hope, a more satisfactory analysisof physical causality.

1. Introduction. Ten years ago, I offered an account of causality in-volving causal processes, as the means by which causal influence is trans-mitted (Salmon 1984, chap. 5), and causal forks, as the means by whichcausal structureis generated and modified (chap. 6). Causal forks comein two main varieties, interactive forks and conjunctive forks. (Perfectforks are a limiting case of both of these types.) Interactive forks are usedto define causal interactions. Causal processes and causal interactions arethe basic causal mechanisms according to this approach.Although causalinteractions are more fundamentalthan causal processes on this view, forvarious heuristic reasons I introduced causal processes before causal in-teractions. (I fear that the heuristic strategy was counterproductive.)Theidea was to present a "process theory" of causality that could resolve thefundamentalproblem raised by Hume regardingcausal connections. Themain point is that causal processes, as characterizedby this theory, con-stitute precisely the objective physical causal connections which Humesought in vain. The so-called at-at theory of causal propagation enablesus to account for the transmission of causal influence in a manner thatdoes not conflict with (what I take to be) Humean empirical strictures.

To implement this programit is necessary to distinguish genuine causalprocessesfrompseudoprocesses. The notion of a process(causalor pseudo)can reasonably be regarded as a primitive concept that can be made suf-ficiently clear in terms of examples and informal descriptions, such aswhat B. Russell (1948) called "causal ines";however, even thoughRussell*Received May 1993; revised July 1993.tI should like to express my sincere thanks to Philip Kitcher, Allen Janis, and an anon-ymous referee for extremely valuable comments on this paper.tSend reprint requests to the author, Philosophy Department, University of Pittsburgh,Pittsburgh, PA 15260, USA.

Philosophy of Science, 61 (1994) pp. 297-312Copyright ? 1994 by the Philosophy of Science Association.

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WESLEY C. SALMONused the word "causal", he did not make a careful distinction betweencausal processes and pseudo processes. Priorto this distinction, the con-cept of process carries no causal involvements. If one thinks in terms ofrelativity theory and Minkowski spacetime diagrams, processes can beidentified as spacetime paths that exhibit continuity and some degree ofconstancy of character. These spacetime paths and their parts may betimelike, lightlike, or spacelike.Processes, whether causal or pseudo, often intersect one another inspacetime; in and of itself spacetime intersection is not a causal concept.Looking at intersections, we need criteria to distinguish genuine causalinteractions from mere spacetime intersections. The basic theses are (1)that causal processes could be distinguished from pseudo processes interms of their capacity to transmitmarks, and (2) that causal interactionscould be distinguished from mere spacetime intersections in terms of mu-tual modifications-changes that originate at the locus of the intersectingprocesses and persist beyond thatplace. In orderto explain what is meantby transmittinga mark it is necessary to explain what is involved in in-troducing a mark. Introducinga mark is a causal concept, so it needs tobe explicated; this is done in terms of the notion of a causal interaction.Causal interactions are explicated without recourse to other causal con-cepts. Contrary to the heuristic order, causal interactions are logicallymore basic than causal processes.This account of causality has certain strong points and certain defects,and it has been subjectedto severe criticism by a number of philosophers.Some of the criticisms are well founded; some are based on misinterpre-tations. In this paper I address these difficulties. First, I will try to clearaway the misinterpretations. Second, I will attemptto show how the ac-count can be modified so as to remove the genuine shortcomings. In thislatter endeavor I rely heavily on work of P. Dowe (1992a,b,c). The resultis, I believe, a tenable-more tenable?-theory of physical causation.2. The Circularity Charge. Dowe (1992c) claims thatthe foregoing ac-count is circular and discusses similar criticisms made by several authors.As a basis for his discussion of this and other criticisms he advances,Dowe formulates six propositions to characterize my position:

D-I. A process is something which displays consistency of char-acteristics.D-II. A causal process is a process which can transmita mark.

D-III. A mark is transmitted over an interval when it appears ateach spacetime point of that interval, in the absence of in-teractions.

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CAUSALITY WITHOUT COUNTERFACTUALSD-IV. A mark is an alteration to a characteristic, introduced by asingle local interaction.D-V. An interaction is an intersection of two processes.D-VI. A causal interaction is an interaction where both processesare marked. (Ibid., 200; "D" stands for Dowe)

In order to evaluate various criticisms we must examine the foregoingpropositions. Dowe's concern with circularityfocuses on D-IV and D-VI;takentogether,he argues, they contain a circularity.StatementsD-I throughD-IV are acceptable just about as they stand;only D-IV requires a bit ofmodification, namely, the substitutionof "intersection" for "interaction".Propositions D-V and D-VI require more serious revision; in fact, D-Vshould be deleted, while D-VI should be modified to read, "A causalinteraction is an intersection in which both processes are markedand themarkin each process is transmittedbeyond the locus of the intersection".There are two crucial points. First, in my terminology "causal interac-tion" and "interaction"are synonymous; there are no such things as non-causal interactions. There are, of course, noncausal intersections. Sec-ond, for an intersection o qualify as a causal interaction, he modificationsthat originate in the intersection must persist beyond the place at whichthe intersection occurs.Let us rewrite the foregoing propositions, taking the required modifi-cations into account and rearrangingthe order. For the sake of furtherclarityI substitutea differentproposition or D-V. Let "S"stand for Salmon;in each case Dowe's counterpart s indicated parenthetically:S-I. A process is something that displays consistency of charac-teristics (D-I).S-II. A mark is an alteration to a characteristic that occurs in a

single local intersection (D-IV).S-III. A markis transmittedover an intervalwhen it appearsat eachspacetime point of that interval, in the absence of interactions(D-III).S-IV. A causal interaction is an intersection in which both pro-cesses are marked (altered) and the mark in each process istransmittedbeyond the locus of the intersection (D-VI).S-V. In a causal interaction a mark is introduced into each of the

intersecting processes. (This substitute for D-V can be con-strued as a definition of "introduction of a mark".)S-VI. A causal process is a process that can transmita mark(D-II).

This revised list of propositions involves certainproblems to which I will

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return,but it does not suffer from circularity. We assume that such spa-tiotemporal concepts as intersection and duration are clear. We assume(for the moment, but see section 3) that we know what it means to saythat a property of a process changes, or that two characteristics of pro-cesses differ from one another. Proposition S-I indicates what counts asa process. Material particles in motion, light pulses, and sound wavesare paradigmexamples. PropositionS-II introducesthe concept of a mark.Notice that a mark is simply a modification of some kind; it need notpersist. When the shadow of an automobile traveling along a road witha smooth berm encounters a signpost, its shape is altered, but it regainsits former shape as soon as it passes beyond the post. It was marked atthe point of intersection, but the mark vanishes immediately. Notice alsothat a pair of causal processes can intersect without constituting a causalinteraction;for example, light waves that intersect are said to interfere inthe region of intersection, but they proceed beyond as if nothing hadhappened.StatementS-III is a key proposition;it characterizesthe notion of trans-mission. Even if we give up the capacity for mark transmission as a fun-damental explication of causal process-as I will do-the concept oftransmission remains crucial (see sec. 7, def. 3 below). Statement S-IVis also a key proposition because it introduces the most basic notion-causal interaction. It says that a causal interaction is an intersection ofprocesses in which mutual modifications occur that persist beyond thelocus of intersection.

Proposition S-V, in contrast, is trivial; it defines "introduction of amark", a concept we will not need. Proposition S-VI is one of the centraltheses of my 1984 theory; it is one I am preparedto abandonin the lightof Dowe's alternativeproposal. I return to this issue in section 7.3. The Problem of Vagueness. Dowe (1992c, 201-204) justifiablycomplainsthatin my discussionsof markingandmarktransmission Salmon1984, chap. 5) I used such terms as "characteristic"and "structure"with-out specifying their meanings. He suggests that introduction of the con-cept of a nonrelational property might have clarified the situation. He isright. Somewhat ironically, in an earlier chapterof the same book (ibid.,60-72), I worked hard to precisely characterize the concept of objectivehomogeneity of reference classes, and dealt with the kind of problem thatcomes up in the discussion of marks. Unfortunately, I neglected to carrythe same type of consideration explicitly into the context of marking. Thekey concept is that of an objectively codefined class (ibid., 82, def. 2),which is explicated in terms of physically possible detectors attached toappropriatekinds of computers that receive carefully specified types ofinformation. It is possible to ascertain, on the basis of local observa-

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CAUSALITY WITHOUT COUNTERFACTUALStions-detections-whether an entity possesses a given propertyat a par-ticular time. Since, in scientific contexts, we often detect one propertyby observing another, it must be possible in principle to construct a com-puter to make the determination. For example, when we measure tem-peratureby using a thermocouple, we actually read a galvanometer todetect an electrical current. The computer to which the explication refersmust be able to translate the galvanometer reading into a temperaturedetermination, on the basis of laws concerning the electrical outputs ofthermocouples, but without receiving information from otherphysical de-tectors. Notice that this explication is physical, not epistemic. This kindof definition would easily suffice to rule out such properties as being theshadow of a scratched car (Kitcher 1989, 463) or being a shadow thatis closer to the Harbour Bridge than to the Sydney Opera House (Dowe1992c, 201), as well as properties such as grue (Goodman 1955).No basic problems concerning the natureof marks arise in connectionwith the distinction between causal processes and pseudo processes thatcannot be handled through the use of the techniques involved in expli-cating objectively codefined classes. As Dowe (1992c, 203) notes, I madea remarkto this effect in Salmon (1985), but regrettably (due to severespace limitations) I neglected to give details. However, since I am aboutto abandon the mark criterion altogether, there is no need to pursue thequestion here.4. Statistical Characterization of Causal Concepts. In an illuminatingdiscussion of the possibility of characterizingcausal concepts in statisticalterms, Dowe (1992c, 204-207) voices the opinion that this enterprise ishopeless. He quotes my remark, "I now think that the statistical char-acterization is inadvisable" (Salmon 1984, 174, n. 12), correctly notingthat it expresses agreement with his thesis. Citing the paucity of reasonsgiven in my note, he offers reasons of his own. In a brief paper (Salmon1990), I attemptto spell out reasons of my own. As nearly as I can tell,Dowe and I have no basic disagreement on this issue.5. Counterfactuals. I have frequently used the example of a rotatingspotlight in the center of a circular building to illustrate the differencebetween causal processes and pseudo processes. A brief pulse of lighttraveling from the beacon to the wall is a causal process. If you place ared filter in its path the light pulse becomes red and remains red from thepoint of insertion to the wall without any further intervention. The spotof light that travels around the wall is a pseudo process. You can makethe white spot red by intervening at the wall where the light strikes it,but without further local intervention it will not remain red as it passes

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beyond the point of intervention. Thus, causal processes transmit marksbut pseudo processes do not.The untenability of this characterization was shown forcefully by N.Cartwright(in conversation) by means of a simple example. Suppose thata few nanoseconds before a red filter at the wall turns the moving spotred someone places a red lens on the rotating beacon so that, as the spotmoves, it remains red because of the new lens on the beacon. In such acase, the spot turns red due to a local interaction and remains red withoutany additional local interactions. With or without the intervention at thewall, the spot of light moving around the wall would have been red fromthat point on. Consideration of such cases requireda counterfactual for-mulation of the principle of mark transmission. I had to stipulate, in ef-fect, that the spot would have remained white from that point on if therehad been no local marking(Salmon 1984, 148). In Cartwright'sexample,the spot would have turnedred anyhow, regardless of whether any mark-ing had occurred at the wall.In an extended and detailed discussion of scientific explanation,Kitcherarticulatesa penetratingcritiqueof my causal theory (1989, sec. 6), mak-ing heavy weatherover the appeal to counterfactuals. He summarizes thisaspect of his critique as follows:

I suggest that we can have causation without linking causal pro-cesses. .. . What is critical to the causal claims seems to be the truthof the counterfactuals, not the existence of the processes and inter-actions. If this is correct then it is not just that Salmon's account ofthe causal structureof the world needs supplementing through theintroductionof more counterfactuals.The counterfactualsare the heartof the theory, while the claims about the existence of processes andinteractionsare, in principle, dispensable. Perhapsthese notions mayprove useful in protecting a basically counterfactual theory of cau-sation against certain familiar forms of difficulty (problems of pre-emption, overdetermination, epiphenomena, and so forth).* But, in-stead of viewing Salmon's account as based on his explications ofprocess and interaction, it might be more revealing to see him asdeveloping a particular kind of counterfactual theory of causation,one that has some extra machinery for avoiding the usual difficultiesthat beset such proposals. [*Kitcher's note: See Lewis (1973), bothfor an elegant statement of a counterfactual theory of causation andfor a survey of difficult cases. Loeb (1974) endeavors to cope withthe problem of overdetermination.] (1989, 472)

When H. Reichenbach proposed his mark method, he thought it couldbe used to determine a time direction ([1928] 1957, 136-137). This wasa mistake, as A. Griinbaum 1963, 180-186) has shown. However, draw-

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ing upon suggestions offered in Reichenbach (1956, sec. 23) concerningthe markmethod and causal relevance, I concluded that the markmethodprovideda criterion or distinguishingbetween causal processes and pseudoprocesses, without any commitment to time direction (earlier-later). Thatis a separate problem (see Dowe 1992b). It has always been clear that aprocess is causal if it is capable of transmittinga mark, whether or notit is actually transmittingone. The fact that it has the capacity to transmita mark is merely a symptom of the fact that it is actually transmittingsomething else. That other something I described as information, struc-ture, and causal influence (1984, 154-157).When the mark criterion was clearly in trouble because of counterfac-tual involvement, it should have been obvious that the markmethod oughtto be regarded only as a useful experimental method for tracing or iden-tifying causal processes (e.g., the use of radioactive tracers) but that itshould not be used to explicate the very concept of a causal process.Dowe took the crucial step. He pointed out that causal processes transmitconserved quantities; and by virtue of this fact, they are causal. I hadcome close to this point by mentioning the applicability of conservationlaws to causal interactions, but did not take the crucial additional step(ibid., 169-170). Dowe's theory is not counterfactual.6. Dowe's Conserved Quantity Theory. Dowe's proposed conservedquantitytheory is beautiful for its simplicity. It is based on just two def-initions (1992c, 210):

DEFINITION1. A causal interaction is an intersection of world-lineswhich involves exchange of a conserved quantity.This definition is a substitute for my much more complex and contortedprinciple CI (for causal interaction) which was heavily laden with coun-terfactuals (Salmon 1984, 171).In discussing interactions it is essential to keep in mind the fact thatwe are dealing with conserved quantities. In an interaction involving anexchange of momentum, for example, the total momentum of the out-going processes must be equal to that of the incoming processes. Thispoint is importantin dealing with certain kinds of interactions in whichthree or more processes intersect in virtually the same spacetime region.For example, a solidly hit baseball and an atmospheric molecule, saynitrogen, strike a glass window almost simultaneously. It may be tempt-ing to say that the baseball caused the window to shatter, not the nitrogenmolecule, because the window would not have shatteredif it had not beenstruck by the baseball. But this analysis is unacceptable if we want toavoid counterfactuals.We should say instead that, in the interaction constituted by the nitro-

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gen molecule and the shattering window, momentum is not conserved.Take the window to be at rest; its linear momentum is zero. The linearmomentum of the nitrogen molecule when it strikes the window is notzero, but fairly small. The total linear momentum of the pieces of theshattered window after the collision is enormously greater than that ofthe incoming molecule. In contrast, the total linear momentum of thebaseball as it strikes the window is about equal to the momentum of thepieces of glass and the baseball after the collision. So if we talk aboutcauses and effects, we arejustified in saying that the window was brokenby the collision with the baseball, not by the collision with the nitrogenmolecule. With these considerations in mind, I think we can say thatDowe's definition 1 is free of counterfactuals, and is acceptable as itstands.

DEFINITION 2a. A causal process is a world-line of an object whichmanifests a conserved quantity.As we will see, definition 2 requires some furtherwork-hence the des-ignation 2a.In his elaboration of the foregoing definitions Dowe mentions mass-energy, linear momentum, angularmomentum, and electric charge as ex-amples of conserved quantities. He explains the meanings of other terms:

An exchange means at least one incoming and at least one outgoingprocess manifest a change in the value of the conserved quantity."Outgoing"and "incoming" are delineated on the spacetime diagramby the forward and backward light cones, but are essentially inter-changeable. The exchange is governed by the conservation law. Theintersection can therefore be of the form X, Y, X or of a more com-plicated form. An object can be anything found in the ontology ofscience (such as particles, waves or fields), or common sense. (1992c,210)

Dowe offers several concrete examples of causal interactions and causalprocesses involving electric charge and kinetic energy, and a pseudo pro-cess not involving any conserved quantity(ibid., 211-212). I made pass-ing mention of two sorts of interaction which, to my great frustration, Idid not know how to handle (1984, 181-182). A Y-typeinteractionoccurswhen a single process splits in two, such as radioactive decay of a nucleusor a hen laying an egg. A A-type interaction occurs when two separateprocesses merge, such as the absorption of a photon by an atom or theconsumption of a mouse by a snake. Dowe points out that his conservedquantity theory handles interactions of these two kinds.

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CAUSALITY WITHOUT COUNTERFACTUALS7. Conserved Quantities and Invariants. A curious ambiguity arisesnear the conclusion of Dowe (1992c). D. Fair (1979) had proposed atheory of causality in terms of transmission of energy which Dowe crit-icizes on the basis of severalconsiderations,"Anotheradvantage[of Dowe'stheory] concerns Fair's admission that energy is not an invariant andtherefore will vary according to the frame of reference. . . . On our ac-count, however, cause is related to conserved quantities and these areinvariant, for example, energy-mass, energy-momentum, and charge"(Dowe 1992c, 214). Up to this point Dowe has formulatedand discussedhis theory entirely in terms of conserved quantities, and the concept ofan invariant has not entered. The terms "conserved quantity" and "in-variant" are not synonymous. To say that a quantity is conserved (withina given physical system) means that its value does not change over time;it is constant with respect to time translation. To say that a quantity isinvariant (within a given physical system) means that it remains constantwith respect to change of frame of reference.Consider linear momentum, which Dowe identifies as a conservedquantity(ibid., 210). We have a law of conservation of linear momentum;it applies to any interactiondescribed with respect to any particularframeof reference, for instance, the "lab frame" in which an experiment isconducted. Within any closed system the total quantityof linear momen-tum is constant over time. If you switch to a different frame of referenceto describe the same physical system, the quantity of linear momentumwill again be constant over time, but not necessarily the same constantas in the lab frame. On Einstein's famous train, for instance, the linearmomentum of the train is zero, but in the frame of the ground observerit has a great deal of linear momentum. Linear momentum is a conservedquantity, but not an invariant.Its value differs from one frame to another.Electric charge is an invariant;the electric charge of the electron has thesame value in any frame of reference. It is also a conserved quantity.Kinetic energy-which Dowe mentions in one of his examples (ibid.,212, example 3)-is neither a conserved quantity nor an invariant. Ininelastic collisions it is not conserved, and its value changes with changesof reference frame. This example is easily repaired,however, by referringto linear momentum instead of kinetic energy.The question arises as to whether we should require causal processesto possess invariantquantities, or whether conserved quantities will suf-fice. At first blush it would seem that conserved quantities will do. Weshould note, however, that causality is an invariant notion. In specialrelativity the spacetime interval is invariant; if two events are causallyconnectable in one frame of reference they are causally connectable inevery frame. Spacelike, lightlike, and timelike separationsare invariants.If two events are causally connected in one frame of reference they are

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causally connected in all frames. Since we are attempting to explicateframe-independentcausal concepts it seems reasonable to insist that theexplicans be formulated in frame-independent terms (see Miihlh6lzer,forthcoming).If, however, we rewrite Dowe's definition 2a as follows, substituting"invariant"for "conserved",

DEFINITIONb. A causal process is a world-line of an object that man-ifests an invariant quantity,we find ourselves in immediate trouble. Consider, for example, a shadowcast by a moving cat in an otherwise darkened room when a light is turnedon for a limited period. This shadow is represented by a world-line withan initial point and a final point. The spacetime interval between thesetwo endpoints is an invariantquantitythat is manifested by a pseudo pro-cess. Any pseudo process of finite duration manifests such an invariantquantity. Definition 2b is patently unacceptable.The main trouble with definition 2b may lie with the term "manifests",for with its use we seem to have abandoned one of the most fundamentalideas about causal processes, namely, that they transmit something (e.g.,marks, information, causal influence, energy, electric charge, momen-tum). A process, causal or pseudo, cannot be said to transmit its invariantspacetime length. A necessary condition for a quantity to be transmittedin a process is that it can meaningfully be said to characterize or be pos-sessed by that process at any given moment in its history. A proton, forexample, has a fixed positive electric charge-which, as already noted,is both conserved and invariant-and it has this charge at every momentin its history. Thus, it makes sense to say that the charge of a particularproton changes or stays the same over a period of time. Perhaps, then,we could reformulate definition 2b as follows:

DEFINITIONc. A causal process is a world-line of an object that man-ifests an invariantquantityat each momentof its history (each space-time point of its trajectory).We should also note that "manifests" contains a possibly serious ambi-guity. A photon, for example, has an electric charge equal to zero. Dowe want to say thatit manifests thatparticularquantityof electric charge?Considerfirst the claim that a neutralhydrogen atom manifests an elec-tric charge of zero. This seems unproblematicbecause the atom is com-posed of two parts, a proton and an electron, each of which has a nonzerocharge. Since the atom can be ionized we can separate the two chargedparticles from one another. The neutron is also unproblematic for tworeasons. First, it is thoughtto be composed of threequarks,each of whichhas a nonzero charge, but separating them is extremely difficult if not

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impossible. Second, a free neutron has a half-life of a few minutes, andwhen it decays it yields two charged particles, a proton and an electron(plus an unchargedantinutrino);unlike the hydrogen atom, however, theneutronis not composed of a proton and an electron. The photon is moredifficult. Under suitable circumstances (e.g., near a heavy atom) a high-energy photon will vanish, yielding an electron-positron pair, each mem-ber of which has a nonzero charge. Thus energetic photons may be saidto have zero electric charge, an attributionwhich can be extended to lessenergetic photons. Any entity that can yield products with nonzero elec-tric charge can be said to manifest an electric charge of zero. However,this kind of principlemight not hold for all invariantsthat a process mightmanifest. The importantpoint is that we must block the assertion that ashadow is an entity thatmanifests an electric charge (whose value is zero)and similar claims. Let us make an additional modification:

DEFINITION 2d. A causal process is a world-line of an object that man-ifests a nonzero amount of an invariant quantity at each moment ofits history (each spacetime point of its trajectory).We need not fear for the causal status of photons on this definition; theymanifest the invariantspeed c.When we speak in definition 2d of a nonzero amount of a given quan-tity, it must be understoodthat this refers to a "naturalzero" if the quan-tity has one. Although temperature s neither a conserved quantitynor aninvariant, it furnishes the easiest exemplification of what is meant by a"naturalzero". The choice of the zero point in the Fahrenheitand Celsiusscales is arbitrary.The fact that water freezes at 0? C and boils at 100? Cdoes not remove the arbitrariness,since the scale is referredto a particularsubstance as a matter of convenience. In contrast 0? K on the absolutescale is a naturalzero, because it is the greatest lower bound for tem-peraturesof any substance under any physical conditions. Applying sim-ilar considerations, we can argue that electric charge has a "naturalzero"even though it can assume negative values. An entity that has a chargeof zero esu (electrostatic units) is not attractedor repelled electrostaticallyby any object that has any amount of electric charge. When it is broughtinto contact with an electroscope the leaves do not separate. It would bepossible (as an anonymousrefereepointedout) to define a quantityelectric-charge-plus-seventeen, which is possessed by photons, shadows, neu-trons, and so on in a nonzero amount. This would be an invariantquan-tity, but it lacks a "natural zero". Given the "naturalzero" from whichit departs, it should be considered inadmissible in the foregoing defini-tion. I believe that all of the quantities we customarily take as conservedor invarianthave "naturalzeroes", but I do not have a general proof ofthis conjecture.

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WESLEY C. SALMONDefinition 2d brings us back, regrettably, to Fair's (1979) account ofcausation in terms of energy transfer, which is open to the objection thatit gives us no basis for distinguishing cases in which there is genuinetransmission of energy from those in which the energy just happens toshow up at the appropriateplace and time. Making reference to the ro-tating beacon in the astrodome, I argued that uniform amounts of radiantenergy show up along the pathway of the spot moving along the wall,and that thereforethe fact thatthe world-line of this spot manifests energyin an appropriatelyregular way cannot be taken to show that the movingspot is a causal process (Salmon 1984, 145-146). Dowe (1992c, 214)

complains that it is the wall ratherthan the spot thatpossesses the energy;however, we can take the world-line of the part of the wall surface thatis absorbingenergy as a result of being illuminated. This world-line man-ifests energy throughoutthe period during which the spot travels aroundthe wall, but it is not the world-line of a causal process because the energyis not being transmitted; t is being received from an exterior source. (IfDowe's objection to this example is not overcome by the foregoing con-siderations, other examples could be supplied.) For this reason, I wouldpropose a further emendation of Dowe's definition:DEFINITION2e. A causal process is a world-line of an object that trans-mits a nonzero amount of an invariant quantity at each moment ofits history (each spacetime point of its trajectory).

This definition introducesthe term "transmits",which is clearly a causalnotion, and which requires explication in this context. I offer the follow-ing modification of my mark transmission principle (MT) (Salmon 1984,148):DEFINITION3. A process transmitsan invariant(or conserved) quantity

from A to B (A # B) if it possesses this quantity at A and at B andat every stage of the process between A and B without any inter-actions in the half-open interval (A, B] that involve an exchange ofthat particular invariant (or conserved) quantity.The interval is specified as half-open to allow for the possibility that thereis an interactionat A that determines the amount of the quantity involvedin the transmission. This definition embodies the at-at theory of causaltransmission (ibid., 147-157), which still seems to be fundamental toour understandingof physical causality. Definition 3 does not involvecounterfactuals.

Speaking literally, the foregoing definitions imply that a causal processdoes not enter into any causal interactions. For example, a gas moleculeconstitutes a causal process between its collisions with other moleculesor the walls of its container. When it collides with another molecule, it

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CAUSALITY WITHOUT COUNTERFACTUALSbecomes another causal process which endures until the next collision.A typical value for the mean free path is 10-7 m, which, though small,is much greater than the size of the molecule. If we consider the lifehistory of such a molecule during an hour, it consists of a very largenumber of causal processes each enduring between two successive col-lisions. Each collision is a causal interaction in which momentum is ex-changed. When we understandthis technical detail, there is no harm inreferringto the history of a molecule over a considerable period of timeas a single (composite) causal process that enters into many interactions.The history of a Brownian particle suspended in the gas is an even moreextreme case, for it is undergoing virtually continuous bombardmentbythe molecules of a gas, but I think it should be conceived in essentiallythe same manner.In many practical situations definition 3 should be considered an ideal-ization. As an anonymous referee remarked, "You'd want to say that thespeeding bullet transmits energy-momentumfrom the gun to the victim,but what about its incessant, negligible interactions with ambient air andradiation?"Of course. In this, and many similar sorts of situations, wewould simply ignore such interactionsbecause the energy-momentum ex-changesare too small to matter.Pragmaticconsiderationsdeterminewhethera given "process" is to be regarded as a single process or a complexnetworkof processes and interactions. In the case of the "speedingbullet"we are not usually concerned with the interactions among the atoms thatmake up the bullet. In dealing with television displays we may well beinterested in the flight paths of individual electrons. In geophysics wemight take the collision of a comet with the earth to be an interactionbetween just two separate processes. It all depends upon the domain ofscience and the nature of the question under investigation. Idealizationsof the sort just exemplified are not unfamiliar in science.There is, however, anothersource of concern. According to Dowe, "Aconserved quantityis any quantityuniversallyconserved accordingto cur-rent scientific theories" (1992c, 210). This formulation cannot be ac-cepted. Parity, for example, was a conserved quantity according to thethen-currenttheories prior to the early 1950s, but in 1956 it was shownby T. D. Lee and C. N. Yang that parity is not conserved in weak in-teractions. According to more recent theories parity is not a conservedquantity. What we should say is that we look to currentlyaccepted the-ories to tell us what quantities we can reasonably regard as conserved.We had good reason to regard parity as a conserved quantity prior to1956; subsequently, we have had good reason to exclude it from the classof conserved quantities. So our current theories tell us what quantities tothink of as conserved; whether or not they are conserved is anotherques-tion.

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WESLEY C. SALMONWe might be tempted to say that conserved quantities are those quan-tities governed by conservation laws, where by "law" we mean either atrue lawlike statement or a lawful regularity in nature. If we were to takethis tack, however, we would free ourselves from the curse of counter-factuals only at the price of taking on the problem of laws. An approachof this sort is given by F. J. Clendinnen (1992), in which he proposes a"nomic dependence" account of causation as an alternativeto D. Lewis'scounterfactualtheory. This may not take us out of the frying pan into thefire, but it does seem to offer anotherhot skillet in exchange for the fryingpan. The hazardcan be avoided, I think, by saying that a conserved quan-

tity is a quantity that does not change. I am preparedto assert that thecharge of the electron is 4.803 x 10 10esu, and that the value is constant.Obviously I could be wrong about its value or about its constancy, butwhat I said in the foregoing sentence is true to the best of my knowledge.If the statement about the electron charge is true, then there is a truegeneralization about the charge of the electron. However, it makes nodifference whether or not that true generalization is lawful; only its truthis at stake. The problem of laws is the problem of distinguishing truelawlike generalizations from other true generalizations. That is a problemwe do not have to face.In discussing the relationship between conserved quantities and laws,I deliberately chose as an example a quantity that is also an invariant.Thus, in fact, I want to stick to the formulation of definition 2e in termsof invariants. I have a furtherreason for this choice. When we ask aboutthe ontological implications of a theory, one reasonable response is tolook for its invariants. Since these do not change with the selection ofdifferent frames of reference-different perspectives or points of view-they possess a kind of objective status that seems more fundamental thanthat of noninvariants.

Apparently, although Dowe's conserved quantity (CQ) theory of cau-sality embodies importantimprovements over my marktransmission the-ory, it is not fully satisfactory as he has presented it. In definitions 1, 2eand 3 I think we have made considerable progress toward an adequatetheory of causality. This is a result, to a large extent, of Dowe's effortsin developing a process theory of causality that avoids the problems ofcounterfactuals with which my former theory was involved. We have, Ibelieve, clean definitions of causal interaction, causal transmission, andcausal processes on which to found a process theory of physical causality.8. Kitcher's Objections. Among the many critiques of my account ofcausality, those of Dowe (1992c) and Kitcher (1989) seem the most pen-etrating and significant. Dowe's discussion is motivated by a desire toprovidean accountof process causalitythat is more satisfactory hanmine.

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CAUSALITY WITHOUT COUNTERFACTUALSAs I have indicated, I think he has succeeded in very large measure, andin the preceding section I tried to improve on his version. Kitcher's mo-tivation is essentially the opposite; he supportsan altogether different ac-count of causality. His thesis is that "the 'because' of causation is alwaysderivative from the 'because' of explanation" (ibid., 477). My view isroughly the opposite, and I think Dowe would agree. Dowe (1992a) as-sesses the difficulties posed by Kitcher and offers his own defense againstthem.Kitcher does not claim to have refuted the causal account of expla-nation, "The aim of this section has been to identify the problems thatthey will have to overcome, not to close the books on the causal ap-proach" (1989, 476). While I do not think that the theory I advocated inSalmon (1984) contained adequateresources to overcome the difficultieshe pointedout, I am inclined to believe that the versiondevelopedherein-leaning heavily on Dowe's work-does have the capacity to do so. Forexample, Kitcher considers the problem of counterfactual entanglement"the most serious trouble of Salmon's project and [one] which I take tothreatenany programthat tries to use causal concepts to groundthe notionof explanation while remaining faithful to an empiricist theory of knowl-edge" (1989, 470). Dowe's primary contribution is to free the conceptof causality from its dependence on counterfactuals. This I consider amajor part of the answer to Kitcher's challenges.Anothercluster of problemsinvolves the concept of a mark(ibid., 463-464). Inasmuch as I have now abandoned the mark transmission approachand substituted the invariant (or conserved) quantity transmission view,the difficultiesconcerningmarkshave been bypassed. In addition,Kitcherpoints to the problem of sorting out in complex situations which inter-actions are relevant and which are not pertinent (ibid., 463). Definition3 goes some distance in responding to this problem inasmuch as it iden-tifies a particular nvariant(or conserved) quantitythat is involved in thetransmission. So Kitcher's misgivings-well taken regarding my (1984)treatment-have been circumvented.

REFERENCESClendinnen, F. J. (1992), "Nomic Dependence and Causation", Philosophy of Science 59:341-360.Dowe, P. (1992a), "An Empiricist Defence of the Causal Account of Explanation", In-ternational Studies in the Philosophy of Science 6: 123-128.. (1992b), "Process Causality and Asymmetry", Erkenntnis 37: 179-196.. (1992c), "Wesley Salmon's Process Theory of Causality and the Conserved Quan-tity Theory", Philosophy of Science 59: 195-216.Fair, D. (1979), "Causation and the Flow of Energy", Erkenntnis 14: 219-250.Goodman, N. (1955), Fact, Fiction, and Forecast. Cambridge, MA: HarvardUniversityPress.Griinbaum, A. (1963), Philosophical Problems of Space and Time. New York: Alfred A.Knopf.

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312 WESLEY C. SALMONKitcher, P. (1989), "Explanatory Unification and the Causal Structure of the World", inP. Kitcher and W. Salmon (eds.), Minnesota Studies in the Philosophy of Science.

Vol. 13, ScientificExplanation. Minneapolis: University of Minnesota Press, pp. 410-505.Lewis, D. (1973), "Causation", Journal of Philosophy 70: 556-567.Loeb, L. (1974), "CausalTheories and Causal Overdetermination",Journal of Philosophy71: 525-544.Miihlholzer, F. (forthcoming), "Scientific Explanation and Equivalent Descriptions", inW. Salmon and G. Wolters (eds.), Logic, Language, and the Structure of ScientificTheories. Pittsburgh: University of Pittsburgh Press.Reichenbach, H. (1956), The Direction of Time. Berkeley and Los Angeles: University ofCalifornia Press.. ([1928] 1957), The Philosophy of Space and Time. Originally published as Phi-losophie der Raum-Zeit-Lehre (Berlin: Walter de Gruyter). New York: Dover.Russell, B. (1948), HumanKnowledge: Its Scope and Limits. New York:Simon & Schuster.Salmon, W. (1984), Scientific Explanationand the Causal Structureof the World. Princeton:Princeton University Press..(1985), "Conflicting Concepts of Scientific Explanation", Journal of Philosophy82: 651-654.. (1990), "CausalPropensities: Statistical Causality vs. Aleatory Causality", Topoi9: 95-100.

salmon 1994 pos causality without counterfactuals

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