M. THALOS IN FAVOR OF BEING ONLY HUMEAN (Received in revised form 12 May 1997) ABSTRACT. The twin conceptions of (1) natural law as causal structure and (2) explanation as passage from phenomenon to cause, are two sides of a certain philosophical coin, to which I shall offer an alternative – Humean – currency. The Humean alternative yokes together a version of the regularity conception of law and a conception of explanation as passage from one regularity, to another which has it as an instance but of which it is not itself an instance. I will show that the regularity conception of law is the basis of a distinguished branch of physical mechanics; thus the Humean conception of law, like its better-loved rival, enjoys the support of a venerated tradition in mechanical theory – in fact, that strand which culminates in quantum theory. I shall also offer an account of explana- tory asymmetry, a natural companion to the Humean conception of explanation as passage from one regularity to another of greater scope, as an alternative to van Fraassen’s unsatisfactory account. My account of asymmetry is just as free of reliance on context as it is free of reliance on cause. I shall thus proclaim that explanatory asymmetry is at once a reality deserving of philosophical treatment – one not to be given over to the care of psychology or linguistics – and at the same time susceptible of an account worthy of Hume. In speaking of cause and effect we arbitrarily give relief to those elements to whose connection we have to attend in the respect in which it is important to us. [But t]here is no cause nor effect in nature; nature has but an individual existence; nature simply is. – Ernst Mach 1 1. INTRODUCTION There are in circulation two well-known conceptions of natural law. I shall refer to the first of these as the regularity conception or – as it is an article of Hume’s estate – the Humean conception. According to it, laws of nature are nothing but regularities (in Hume’s terms, “constant conjunctions”) among observable natural quantities. 2 I intend to defend this conception of natural law, and to do so in a manner that both refreshes and advances the debate Hume inaugu- rated. For Hume continues to inspire present-day empiricists in the direction of a certain, very healthy, form of skepticism; but he gives Philosophical Studies 93: 265–298, 1999. c 1999 Kluwer Academic Publishers. Printed in the Netherlands.