What is a Law of Nature? A Humean Answer*

True universal generalisations are often categorised either as accidental truths or as laws. Frequently-cited examples of the former are: 'All my friends speak French', 'All the coins in my pocket on VE Day are nickels', and 'All moas die before reaching the age of 50'. The following, on the other hand, are often supposed, if true, to be laws of nature: 'All metals expand on heating', 'In a closed physical system, the total energy is constant', and Newton's 'Laws'. Views differ about the basis of the intuitive distinction between the two types of generalisation. Some philosophers claim that laws, unlike accidental generalisations, express a 'physical' or 'natural' necessity; others hold that the difference between the two types of generalisation rests in part on some aspect of our personal attitude towards them. The first of these, which I call the objectivist view, has the longer tradition and is probably the more popular; the second is a subjectivist theory, often associated with David Hume. In this paper, I shall first propose a theory to account for some intuitive differences between laws and accidental generalisations and for some characteristic patterns of reasoning connected with each. I shall then consider whether the distinction between the two sorts of generalisation is best interpreted along objectivist, or subjectivist lines. I shall conclude in favour of the latter, on the grounds that the purported notions of physical necessity or nomic connection which objectivists have hitherto advocated are no less obscure than the intuitive idea of lawlikeness they were supposed to explicate.