Questions about causation, laws of nature, probability and chance are part of the domain of both metaphysics and philosophy of science. Saying what causation is, or what it is to be a law of nature, will presumably shed light on some of the most basic aspects of our world. Sorting out what causes, laws and chances are would also shed light on the foundations of both the natural and social sciences, since both of these make use of notions of causation and laws of nature, and reliance on probabilistic reasoning and estimating chances of events is ubiquitous. That is not to say that these are entirely technical notions to be dealt with only in the sciences. Thinking and talking about causes is to be found throughout human endeavour, and to the extent we are interested in making predictions or finding explanations in everyday life, some dim sense of what is physically impossible and what the laws of nature permit also seems to play a role. Even at the high point of suspicion about metaphysics in Anglo-American philosophy in the twentieth century, few could rid themselves of the view that some things cause other things, or that there is something special about the regularities scientists uncover, as opposed to regularities that are entirely coincidental. The notion of causation is also caught up in the notion of explanation, and Lewis is a champion of a distinctive theory about explanation that related explanation to causation.