eighty-second annual meeting american philosophical association, eastern division || two approaches...

Journal of Philosophy, Inc.

Two Approaches to ExplanationAuthor(s): Philip KitcherSource: The Journal of Philosophy, Vol. 82, No. 11, Eighty-Second Annual Meeting AmericanPhilosophical Association, Eastern Division (Nov., 1985), pp. 632-639Published by: Journal of Philosophy, Inc.Stable URL: http://www.jstor.org/stable/2026419 .

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632 THE JOURNAL OF PHILOSOPHY

contending that historical study replaces or takes over the doing of philosophy. But philosophizing out of history, still occurs in history, and, for better or worse, seems to feed on historical texts or previous historical thinkers. Those who claim to have found the Truth in 1985, will, I am sure, report their findings in terms of its links with both the personal history of the truth-finder, and the history of the problem-solvers the truth-finder has built upon or combatted. We are historical beings, and cannot escape our own history or our cultures. So, then, shouldn't we try to profit from it, both by following Socrates' injunction to know ourselves, and the further injunction, to know as best we can how we got to our pres- ent intellectual situation, personally and culturally, and what we can do about it?

RICHARD H. POPKIN

Washington University, St. Louis

TWO APPROACHES TO EXPLANATION*

W SESLEY Salmon's rich study of explanation and causa- tion provides the most articulate version of the thesis that scientific explanation consists in fathoming the

causal structure of the world. Because Salmon provides balanced evaluations of rival views, his discussion deserves the same attention that philosophers have rightly lavished on Carl Hempel's essays. I shall consider three central themes in Salmon's book. A contrast between two general approaches to explanation will emerge.

Salmon claims that statistical explanation is fundamental and that its neglect by philosophers until the 1960s represented a failure to confront twentieth-century science (24, 55). The claim rests on two observations: quantum mechanics (QM) seems to be indeterministic (53, 111); many areas of science offer a statistical treatment of macroscopic phenomena (26, 47).

There is a natural response to this. Genuine explanation is deductive. Insofar as QM offers only probabilistic conclusions, the phenomena are not explained. (Thus, for example, QM provides * To be presented in an APA symposium on Wesley Salmon's Scientific Explanation and the Causal Structure of the World, December 30, 1985. Bas van Fraassen will be co-symposiast, and Wesley Salmon will respond. See this JOURNAL, this issue, 639-651 and 651-654, respectively, for their contributions.

I would like to thank Wesley Salmon for helpful comments on an earlier draft. All parenthetical page references are to Scientific Explanation and the Causal

Structure of the World (Princeton, N.J.: University Press, 1984). 0022-362X/85/821 1/0632$00.80 ? 1985 The Journal of Philosophy, Inc.



SALMON ON EXPLANATION AND CAUSALITY 633

no explanations of individual events of radioactive decay.) In some areas of science that study macroscopic phenomena, statistical claims turn up in the context of testing. Where they occur in explanatory contexts, they are merely place-holders for hypothetical deductive explanations which scientists are unable to construct in detail.

Salmon correctly dismisses this response as too simple (52, 111, 190). I suggest that analysis of varieties of explanation-seeking questions enables us to capture some of its insights and to honor some plausible principles that Salmon is driven to reject (1 14-120).

There are degrees of scientific understanding, and, equally, degrees of ununderstanding. An explanation is ideally complete if it eliminates all our ununderstanding. An account may advance our understanding without being ideally complete. We may have to settle for less. For there may be phenomena for which no ideally complete explanation is possible. If so there is a conflict between our ideals of understanding and what nature allows.

Requests for explanation are typically fuzzy. An explanation- seeking question "Why p?" usually covers several more precise queries. In studying scientific explanation it is helpful to identify exactly the questions we can answer. Where we can answer some but not all the precise queries intended in our explanation-seeking question, it is wrong both to dismiss the science as nonexplanatory and to hail it as giving ideally complete explanations.

Three examples will serve as illustrations and will help us to see the role(s) of probabilistic notions in explanation.

(a) Electrons approach a potential barrier. For each electron, the probability that it will be reflected is 0.9, the probability that it will tunnel through is 0.1. Consider two electrons, el, and e2. ei is reflected; e2 tunnels through. Can we explain these events?

QM can answer the following questions: "How is it possible for ei to be reflected?", "What is the probability of el being reflected?", "Why does ei have this probability of being reflected?" (and ana- logues for e2). QM cannot answer the questions: "Why was it ei rather than e2 that was reflected?", "Why was it e2 rather than ei that tunneled through?". It happened that way by chance.

We can avoid a dilemma Salmon poses on several occasions (110, 120, 277). There is no need to deny that QM advances our understanding of individual events-for it answers some questions. Nor need we give up Bas van Fraassen's idea that an answer to "Why p?" should show why p rather than pi, where pi belongs to the intended contrast class. That idea is a constraint on an ideally complete answer. QM does not provide ideally complete explanations of electron interactions with potential barriers.




(b) Salmon appeals to an example from genetics to undermine Hempel's high-probability requirement and its descendants (88, 109). A breeding experiment on pea plants produces a filial population in which 0.75 of the plants have red blossoms, 0.25 white. Let b1, be a plant with red blossoms, b2 a plant with white blossoms. Can we explain why bi has red blossoms? Supposedly, we "can answer that it comes from this population, and therefore it has a fairly high probability of being red" (86). The analogous probability claim would fail for b2. But, as Salmon notes, we understand the occurrence of white blossoms equally well.

The example exposes an error that infects many discussions of probabilistic treatments of macroscopic phenomena. Note first that the geneticist's answers to the questions "Why does bi have red blossoms?", "Why does b2 have white blossoms?" are "Because bi has genotype RR or Rr (where R is dominant with respect to r and codes for a molecule ultimately producing red pigment)" and "Be- cause b2 has genotype rr". No appeals to probability enter yet.

But the original fuzzy questions cover further queries. We want to know why the plants have these genotypes. Given classical genetics and background knowledge, we can answer some questions. "How is it possible that a parent population of pea plants with red blossoms produced some offspring with white blossoms?"-The parents were heterozygotes Rr. "Why is the probability that an offspring will have white blossoms 0.25?" -Because white blossoms develop only from rr zygotes, and each parent transmits an r gamete with probability 0.5. We are unlikely to know enough to answer a further question. "Why was the zygote from which b2 developed rr rather than RR or Rr?". Because we do not know the details of the fertilization process that produced that zygote, we can say only that it happened that way by chance. But this is a different "by chance", for there is no reason to think it in principle impos- sible to describe the details of the fertilization in a way that makes no appeal to probabilities.

Once again, there is no reason to reject van Fraassen's idea about the importance of contrastive questions (109). Moreover, here only ignorance, not nature, stands in the way of ideally complete explanation.

(c) Why did the mayor develop paresis? He alone among the townspeople had previously contracted syphilis. But the chance that an individual syphilitic develop paresis is low. Does this show, as Salmon suggests (31/2, 51-53), that we can explain events to which we assign low probabilities?

Once again, let us disambiguate questions. There are some ques-




tions that we can answer completely: "Given that one of the townspeople contracted paresis, why was it the mayor?"- Only syphilitics get paresis, and he was the only syphilitic in town. Other questions are unanswerable. We don't know why it was the mayor rather than some of his luckier fellow syphilitics who contracted paresis.

The explanatory strategy that figures here is common in science. One wants to know why a system is in state X rather than in one of a set of rival states {. . Yi . .}. It is presupposed that the system must be in one of these alternative states, and one shows that the system cannot be in any of the Yi. Contemporary evolutionary theory abounds with examples of a special case: one shows that if the system starts in any of the Yi it ends in X.

My examples are intended to divide and conquer. Probabilities turn up in explanations for three quite different reasons: sometimes we deduce probability values for occurrences and obtain par- tial, but not ideally complete understanding [as in (a)]; sometimes probabilities go proxy for underlying deductive accounts that, in practice, we are unable to specify (b); sometimes they are wrongly imported because we do not make precise the questions we can and cannot answer [(c) and, to some extent, (b)].

Two obvious objections. Does my approach reflect "insidious" Laplacean determinism? (53, 113, 120). I do not presuppose that the universe is deterministic. I claim we have an ideal of complete understanding, involving the ability to answer contrastive questions. Because such questions prove unanswerable in indeterministic settings, the best accounts we can give for those settings are not ideally complete. That does not mean those accounts lack explanatory power.

Doesn't modern physics force us to renounce this ideal? Take any deductive explanation of a macroscopic event. Given QM, it is possible that enough fundamental particles simultaneously enter highly improbable states to defeat the universal principles on which the account relies. If explanations are to involve only true principles, then we must settle for probabilistic descriptions of macroscopic phenomena. The alleged ideal is nowhere attainable.

Reply: since Galileo explained ballistics to the gunners of Venice, scientific explanations have been playing "let's pretend". We ignore common perturbations that would have slight effects (cannonballs traveling through air become point-particles in vacuo) and major potential disruptions with negligible probability (simultaneous decay of millions of uranium nuclei). Here Hempel's high-probability requirement may find a place. We don't explain individual




events by showing that they occur with high probability. We give an idealized treatment of the actual phenomena, offer a deductive account, and appeal to probabilities to show that we are justified in ignoring the factors we omit.

II

,Salmon holds that theoretical explanation is parasitic on the causal explanation of individual events (xi, 19, 276). We explain general regularities by identifying the causal mechanisms that produce the events they cover (227). Often physical regularities are subsumed under more general regularities, as the motions of the planets are subsumed under the regularities described by Newton (92).

An opposing tradition claims that scientific explanation is global rather than local. The sciences are not primarily interested in questions about individual events (with rare exceptions: "Why did the dinosaurs go extinct?"). Causal explanation of particular occurrences recapitulates the ordering derived from the systematization of regularities.

Salmon argues that scientists are often interested in particular occurrences. Citing some genetic experiments by Yuichiro Hiraizumi in which violations of Mendel's rules occurred, he concludes that theoretical science hopes to understand the particular case as well as the general regularity (117/8). But, as with many earlier investi- gations in classical genetics, Hiraizumi's concern was to extend patterns of genetic explanation to cope with all the repeatable possibilities. The experimental results are significant because they call attention to an unsuspected regular possibility, segregation distor- tion, which genetics must accommodate within its explanatory framework.

Recall example (b) above. Genetics has no special interest in the fertilization that produced b2's zygote. But if we discovered that the story is not just a tedious account of the trajectories of various gametes, that there is an underlying regularity consisting in differ- ential propensities of certain kinds of gametes to unite, then we would have a new scientific question.

Further, it is sometimes hard to see how delineating individual causal processes is "the key to theoretical explanation" (xi). Per- haps we understand the kinetic-theoretic explanation of the Boyle- Charles law to show that, for each sample of gas, there are causal processes giving rise to temperature and pressure in such a way that the relationship PV = RT is (approximately) maintained. Other examples are more problematic. Quantum chemistry ex- plains why neon is chemically inert: we derive conditions of stabil- ity of atoms in terms of filling of shells with electrons, exhibit the possibilities of bond formation, and thus reveal why elements (like




neon) whose atoms have their outermost shells filled react with other elements only under special conditions.

The explanation does not seem to be a general description of the causal processes that go on in countless particular samples of neon, but a demonstration of why processes that would antecedently have seemed possible do not occur. [Compare the evolutionary examples mentioned in (c) above.]

Moreover, we should recognize that the pattern of explanation to which I alluded can be instantiated in explaining not only the behavior of the noble gases, but also the bonding properties of all the elements, the structure of the periodic table, and so forth. In- stead of thinking of ourselves as constructing theoretical explanations by drawing on antecedently available ideas about causal processes and underlying mechanisms, we could stand Salmon's picture on its head. We note the explanatory priority of the properties and processes described in our quantum-mechanical premises by appre- ciating the possibility of using them to provide a unified account of the chemical behavior of the elements, where the unification consists in our use of the same pattern of derivation in case after case. Our recognition of an explanatory ordering precedes, and makes possible, our identification of causal relationships.

There is another reason for resisting Salmon's assimilation of theoretical explanation to causal explanation. Something very like (scientific) theoretical explanation occurs in areas that allow no obvious application of causal concepts: formal linguistics and mathematics, for example. Lagrange and Galois explained why apparently unmotivated procedures for solving equations suc- ceeded, offering a unified account of the conditions for solvability. Results about equations are explanatorily dependent on Galois theory. But there is no causal dependency.

III Those who propose to use causal concepts to characterize explanation can either respond to or ignore Hume's worries about causa- tion. Salmon is commendably forthright in facing the Humean challenge (20, 147). He holds that we can formulate causal claims so as to make clear how we know them if we focus on causal processes and causal interactions. But how do we recognize causal processes?

Salmon's answer appeals to Hans Reichenbach's "mark method." "A causal process is capable of transmitting a mark; a pseudo-process is not" (142). Moving cars are causal processes, their shadows pseudo-processes.

The discussion of mark transmission is lucid and sensitive to dif- ficulties. The account of marks themselves seems less satisfactory.




On the most anemic reading, x acquires a mark at t and transmits it after t just in case there is a modification of some characteristic Q of x into Q' at t such that x continues to have Q' after t. This reading allows pseudo-processes full status. When the car grazes the wall its shadow acquires, and continues to have, a new characteristic-being the shadow of a scratched car.

This is cheating, no doubt. But why? What counts as a genuine modification of a characteristic? In the spirit of two of Salmon's principles (148, 171), we might propose that marks on x are produced by interactions with x. Such language drips with the causal idioms whose epistemological basis we hope to fathom. To focus on the second instance, Salmon's own account of causal interactions (171) uses the notion of modification of characteristics.

If the account of marks threatens to become either too weak or to presuppose the notions it is supposed to elucidate, there is also a danger that it will be too strong. Scientists often mark organisms in ways that leave permanent but profoundly different effects. If I inject the right chemical into the cytoplasm of a frog zygote, that chemical may be absent from every cell of the young embryo. Yet, the frog is marked-by developmental abnormalities.

In short, it appears that the notions of causal process, causal interaction, mark, and genuine modification are intricately inter- twined. The Humean challenge is to show how we can gain knowledge of statements involving one of these notions without appealing to statements involving any of the others. Although Salmon has made progress in meeting the challenge, a familiar type of difficulty seems to remain.

IV Salmon's approach to explanation is "bottom up". Explanation consists in identifying causal relations. Causal relations primarily relate individual events; so the explanation of particular occurrences is fundamental. Physics tells us that probabilistic causal relations and probabilistic explanation are the norm. Causal notions are prior to the notion of explanation, and it is possible to give an empiricist account of our causal knowledge.

Hempel's approach to explanation was "top down". Explana- tory concepts were conceived as prior to causal concepts. But the D-N model foundered on its liberality. The asymmetries of explanation invited philosophers to make explicit appeal to causal notions.

I claim that a more radical "top down" approach is possible. Begin from the idea that explanation is directed at an ideal of scientific understanding. We achieve that ideal by giving a unified, deductive systematization of our beliefs. Our views about genuine




properties and explanatory dependence emerge from the project of unifying the regularities we discover in nature. On this approach, theoretical explanation is primary. Causal concepts are derivative from explanatory concepts. In explaining particular events we answer as many questions as we can, drawing on our view of the order of natural phenomena. In some cases, our ideal of understanding may not be completely realizable.

The two approaches depart from Hempel in different directions. Nobody has yet developed a radical "top down" approach with the clarity and thoroughness that Salmon has brought to its "bottom up" rival. My aim has been to suggest that, for all its thoroughness, Salmon's program still faces some research problems and that the case against an alternative view is not entirely closed.

PHILIP KITCHER

University of Minnesota

SALMON ON EXPLANATION* A CCORDING to Wesley Salmon's significant and seminal study, science provides us with understanding of a phe- nomenon exactly to the extent that it provides an expla-

nation, and provides explanation to the extent that it fits the phe- nomenon in a discernible pattern of causal relations. I suppose I disagree on both points; but this statement is misleading, for in fact my agreement with Salmon is much more extensive than our disagreement. The causal order as described by Salmon I can understand without sacrifice of philosophical scruples (though I am less certain that the notion of causality is entirely captured), and the kind of explanation he characterizes is, I think, of central importance, though not the only kind. To bring out our agreements and disagreements more concretely, I shall discuss two topics: the logic or structure of explanation, and the limits of causal modeling.

*Contribution to an APA symposium on Wesley Salmon, Scientific Explanation and the Causal Structure of the World (Princeton, N.J.: University Press, 1984), to be held on December 30, 1985. Philip Kitcher will be co-symposiast, and Wesley Salmon will comment; see this JOURNAL, this issue, 632-639 and 651-654, respectively, for their contributions. Parenthetical page references in the text of this paper are to Salmon's book.

The author wishes to acknowledge the support of the National Science Foundation. 0022-362X/85/8211/0639$01.20 0 1985 The Journal of Philosophy, Inc.



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