LAWS OF NATURE

On the standard view, there are three kinds of facts. First, there are the logical or metaphysical necessities: facts that absolutely could not have been otherwise. These include the fact that triangles have three sides and that either you are now sitting down or it is not the case that you are now sitting down. The rest of the facts are contingent. They divide into two classes: the nomic necessities, which follow from the laws of nature alone, and the accidents, which do not. Among the accidents are that all of the coins in my pocket today are silver-colored and that all solid-gold cubes are smaller than a cubic mile. (For the sake of argument, let’s suppose that these are truths.) The laws, according to our best current science, include that all gold is electrically conductive and that electric charge is conserved. Both laws and accidents are contingent: just as magnetic monopoles could possibly have existed and material bodies could possibly have been accelerated from rest beyond 3 3 108 ms21 (contrary to natural law), so a solid-gold cube larger than a cubic mile could have existed (contrary to accidental fact). Notice that the accidental regularity concerning gold cubes is just as general, universal, and exceptionless as the law that all solid cubes of uranium-235 are smaller than a cubic mile. (Large clumps of U-235 undergo nuclear chain-reactions, as in an atomic bomb.) Notice also that a law may currently be undiscovered (though I can’t give you an example of one of those!) and that, after it has been discovered, it need not be officially called a “law” (as with the axioms of quantum mechanics, Bernoulli’s principle, and Maxwell’s equations). Some things that are still called “laws” (such as Newton’s law of gravity and Bode’s law) may not currently be regarded as genuine laws (or even as facts at all). Philosophers have drawn many distinctions among the laws of nature. Some laws are causal (such as laws governing what happens whenever two chemical substances are combined under certain conditions), whereas others are not (such as conservation laws). Some laws are fundamental; others are derived (such as Galileo’s law that any body falling from rest freely to earth covers a distance proportional to the square of the time it has spent falling). Some laws are deterministic; others are probabilistic – that is, statistical (such as that any atom of beryllium-11 at any moment has a 50

percent chance of decaying over the subsequent 13.81 seconds). Some laws are more theoretical or model-driven, whereas others are more phenomenological. Many laws are instantiated, but some are vacuous (as when a law specifies what would happen if two substances were combined under certain conditions, but in fact, they never are). Some philosophers believe that there are laws of special, or “inexact,” sciences, such as population genetics, ecology, mineralogy, psychology, and economics; that these laws frequently include ceteris-paribus clauses; and that their irreducibility to the laws of physics is responsible for the explanatory autonomy of those scientific fields. Other philosophers believe that such “laws” are either fictions (such as that all human beings have ten fingers), accidents (such as the “frozen accident” of the genetic code), or logical necessities (such as the principle that a creature with greater evolutionary fitness is more likely to reproduce than is a less fit creature), and that the genuine laws require no elastic escape clauses. Laws of nature tie into a host of topics of perennial metaphysical and epistemological interest, including causation, chance, confirmation, counterfactuals, determinism, dispositions, emergence, explanation, models, natural kinds, necessity, properties, reduction, unification, and universals. Some philosophers have even denied the “standard view” (“There are three kinds of facts . . .”) with which I began. Scientific essentialists (such as Ellis 2002) regard laws as metaphysically necessary: it is part of electric charge’s essence that it involves the causal power to exert and to feel forces in accordance with certain particular laws. Cartwright (1983) has argued that some processes are not governed by any laws and that statements of the laws of nature are not even truths – at least, when they are interpreted as describing exceptionless regularities, though perhaps they are true as describing causal powers. Giere (1999) and van Fraassen (1989) contend that the philosophical tradition has been led astray in employing the concept of natural law to rationally reconstruct science. In this chapter, I confine myself to two questions (and even then, I can do little more than ask them). First, what difference does it make, in scientific reasoning, whether some truth is believed to be a law or an accident? Second, what is it about the world that makes some fact a law rather than an accident? Ideally, the answer to the second question should account for the answer to the first question. If these questions cannot be answered satisfactorily within the “standard view,” then perhaps something more radical will be necessary.