Causality

Be that as it may, the Deductive-Nomological model, as well as any attempt to tie causality to laws, faces a rather central conceptual difficulty: the problem of how to characterize the laws of nature. Most Humeans have come to adopt the Regularity View of Laws: laws of nature are regularities. Yet, they have a hurdle to jump: not all regularities are causal. Nor can all regularities be deemed laws of nature. The night always follows the day, but it is not caused by the day. And, though a regularity, it is not a law of nature that all coins in my pocket are euros. So, the Humeans have to draw a distinction between the good regularities (those that constitute the laws of nature) and the bad ones—that is, those that are, as Mill put it, "conjunctions in some sense accidental." Only the former can underpin causality and play a role in explanation. Among the many attempts to distinguish between laws and accidents, the most promising is what may be called the web of laws view. According to this, the regularities that constitute the laws of nature are those that are expressed by the axioms and theorems of an ideal deductive system of our knowledge of the world, which strikes the best balance between simplicity and strength. Whatever regularity is not part of this best system is merely accidental: it fails to be a genuine law of nature. The gist of this approach, which has been advocated by Mill, Frank Ramsey (1903–1930), and David Lewis (1941–2001), is that no regularity, taken in isolation, can be deemed a law of nature. The regularities that constitute laws of nature are determined in a kind of holistic fashion by being parts of a structure. But despite its many attractions, this view does not offer a purely objective account of laws of nature.

A contrary view that has been defended by David Armstrong (b. 1926) is that lawhood cannot be reduced to regularity. Lawhood is said to be a certain necessitating relation among natural properties. An attraction of this view is that it makes clear how laws can cause anything to happen: they do so because they embody causal relations among properties. But the central concept of nomic necessitation is still not sufficiently clear.