David Kellogg Lewis (September 28, 1941 October 14, 2001) was a 20th-century American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years. He has made ground-breaking contributions in philosophy of language, philosophy of mind, metaphysics, epistemology, and philosophical logic. He is probably best known for his controversial modal realist stance: that there exist infinitely many concretely existing and causally isolated possible worlds, of which ours is just one, which are used in the analysis of necessity and possibility.