David Lewis (1941-2001) was Class of 1943 University Professor of Philosophy at Princeton University. His contributions spanned philosophical logic, philosophy of language, philosophy of mind, philosophy of science, metaphysics, and epistemology. In On the Plurality of Worlds, he defended his challenging metaphysical position, "modal realism." He was also the author of the books Convention, Counterfactuals, Parts of Classes, and several volumes of collected papers.

If consequentialism is understood as claiming, at least, that the moral character of an action depends only on the consequences of the action, it might be thought that the difficulty of knowing what all the consequences of any action will be poses a problem for consequentialism. J. J. C. Smart writes that in most cases..

“Perhaps the most common objection to consequentialism is this: it is impossible to know the future…This means that you will never be absolutely certain as to what all the consequences of your act will be…there may be long term bad effects from your act, side effects that were unforeseen and indeed unforeseeable…So how can we tell which act will lead to the best results overall – counting all the results? This seems to mean that consequentialism will be unusable as a (...) moral guide to action. All the evidence available at the time of acting may have pointed to the conclusion that a given act was the right act to perform – and yet it may still turn out that what you did had horrible results, and so in fact was morally wrong. Indeed, if will never be possible to say for sure that any given act was right or wrong, since any event can continue to have further unseen effects down through history. Yet if it is impossible to tell whether any act is morally right or wrong, how can consequentialism possibly be a correct moral theory?”. (shrink)