The Explanatory Role of Irreducible Properties

I aim to reconcile two apparently conflicting theses: 1. Everything that can be explained, can be explained in purely physical terms, that is, using the machinery of fundamental physics, and 2. Some properties that play an explanatory role in the higher-level sciences are irreducible in the strong sense that they are physically undefinable: their nature cannot be described using the vocabulary of physics. I investigate the contribution that physically undefinable properties typically make to explanations in the high-level sciences, and I show that when they are explanatorily relevant, it is in virtue of their extension (or something close) alone. They are irreducible because physics cannot capture their nature; this is no obstacle, however, to physics’ more or less capturing their extension, which is all that it need do to duplicate their explanatory power. In the course of the argument, I sketch the outlines of an account of the explanation of physically contingent regularities, such as the regularities found in most branches of biological inquiry, at the center of which is an account of the nature of contingent, empirical “bridge principles”. Science and philosophy are fighting a battle over reduction, or so it seems. On the one hand, for any “high-level” phenomenon—chemical, biological, psychological, economic—science claims to be able to provide, in the long term if not quite yet, a lower-level explanation, and ultimately a physicallevel explanation. The enormous progress that has been made towards this goal can hardly be ignored. On the other hand, philosophers have recently claimed with increasing confidence that many explanatory properties cited by higher-level sciences—being water, being a gene, being a species, being a belief, being currency—are irreducible. The aim of this paper is to show that both sides may be correct. I will characterize very strong versions of both the scientific and the philosophical claims—strong versions of explanatory physicalism and explanatory irreducibility—and I will argue that there exists an explanatory relevance relation, a conception of the explanatory role played by irreducible properties, that allows the two to coexist. I will provide no arguments for either explanatory physicalism or explanatory irreducibility; I will rather simply suppose that both doctrines are correct and attempt a reconciliation. Further, in contrast with much of the recent literature on reductionism, this paper will not be especially concerned with functionally defined or multiply realizable higher-level properties; the explanatory role I find for irreducible properties can be played by functional and non-functional properties, by multiply and singly realizable properties, alike. 1. Explanatory Physicalism Everything that can be explained, can be explained physically; that is the doctrine of explanatory physicalism. More fancifully, if there were a race that only spoke and thought in the language of fundamental physics, and so could not conceive of non-physical properties, they could understand the world as