Parsimony Arguments in Science and Philosophy ─ A Test Case for Naturalismp

Parsimony arguments are advanced in both science and philosophy. How are they related? This question is a test case for Naturalismp, which is the thesis that philosophical theories and scientific theories should be evaluated by the same criteria. In this paper, I describe the justifications that attach to two types of parsimony argument in science. In the first, parsimony is a surrogate for likelihood. In the second, parsimony is relevant to estimating how accurately a model will predict new data when fitted to old. I then consider how these two justifications apply to parsimony arguments in philosophy concerning theism and atheism, the mind/body problem, ethical realism, the question of whether mental properties are causally efficacious, and nominalism versus Platonism about numbers. For many philosophers, the word “naturalism” immediately conjures up a metaphysical and a methodological thesis. Both concern objects that are “in nature,” meaning things that exist in space and time; the contrast is with the supernatural entities that might be thought to exist “outside” of space and time: 1 Metaphysical Naturalism: The only things that exist are things in nature. Methodological Naturalisms: Science should not postulate the existence of things that are outside of nature. I put an “s” subscript on the second naturalism to mark the fact that it gives advice to science. These two naturalisms are the ones that get trotted out in discussions of evolutionary theory versus creationism. Evolutionists often say that their theory obeys the strictures of methodological naturalism but is silent on the metaphysical question. They further contend that creationism rejects both these naturalisms; here they are helped by creationists themselves, who often express their belief in a supernatural deity and argue that methodological naturalisms is a shackle from which science needs to break free. Although it is worth inquiring further into this interpretation of both evolutionary theory and creationism, 2 that is not my topic 1 The words “inside” and “outside” are used metaphorically here. A more literal formulation is that metaphysical naturalism says that all existing things have spatio-temporal location; supernatural entities, if they exist, do not. 2 Mathematized evolutionary theory quantifies over numbers; if numbers are what Platonists say they are (entities that exist outside of space and time), then evolutionary theory violates methodological naturalism (Sober forthcoming). If intelligent design theory is formulated without specifying whether the postulated designer is a supernatural or a natural being, does it thereby obey methodological naturalism? I discuss this question in Sober (2007). 2 here. Rather, I am interested in a third naturalism. Like the second, it is methodological, but it is aimed at the practice of philosophy, not of science (hence the “p” subscript that I use to label it): Methodological Naturalismp: Philosophical theories should be evaluated by the same criteria that are used to evaluate scientific theories. The popularity among philosophers of this form of naturalism owes a lot to Quine‟s influence. When Quine (1953, 1960, 1963) maintained that philosophy is “continuous” with science, he meant that philosophers should address general questions of ontology in the same way that scientists address more specific questions about what there is. 3 There are some similarities that link science and philosophy that lend a superficial plausibility to naturalismp. For example, scientists and philosophers care, or ought to care, about logical consistency. However, there is another context in which this naturalistic thesis is far from obvious. Scientific theories are often evaluated for how parsimonious they are and the same goes for philosophical theories. Taken at face value, this similarity seems to be grist for the naturalist‟s mill. But is the justification for using parsimony in philosophy really the same as the justification for using parsimony in science? The use of the same word in these two contexts should not lead us to assume that they are. One way to be a naturalistp about parsimony is to embrace a kind of nihilism. Perhaps the use of parsimony to adjudicate between scientific theories has no justification whatever and the same is true of its application to philosophical theories. Parsimony arguments in the two fields are on the same footing, namely none. In this paper I‟ll describe two kinds of parsimony inference that occur in science for which I think this nihilistic assessment is mistaken. Parsimony isn‟t always an optional aesthetic frill; there are important types of scientific argument in which parsimony has a demonstrable epistemic relevance. Given this, the question for naturalismp is how the justification that parsimony has in some scientific contexts bears on the parsimony arguments that philosophers produce. 3 Here is a characteristic passage, from Word and Object: “What distinguishes between the ontological philosopher‟s concern and [the scientist‟s] ... is only breadth of categories. Given physical objects in general, the natural scientist is the man to decide about wombats and unicorns. Given classes, or whatever other broad realm of objects the mathematician needs, it is for the mathematician to say ... whether in particular ... there are any cubic numbers that are sums of pairs of cubic numbers ... The philosopher‟s task differs from the others‟, then, in detail; but in no such drastic way as those suppose who imagine for the philosopher a vantage point outside the conceptual scheme that he takes in charge. There is no such cosmic exile (Quine 1960, p. 275).” It is curious that the alternative to naturalismp that Quine describes is one in which philosophers have a vantage point that is outside of the conceptual scheme that they in fact use. Surely rejecting naturalismp involves no such commitment. 4 Just as naturalisms does not entail metaphysical naturalism, so metaphysical naturalism does not entail naturalismp. Metaphysical naturalists may want to appeal to parsimony, both in science and in philosophy, but it is a further question whether they should maintain that the same concept of parsimony applies in the two domains.