David "Kellog" Lewis was a well-respected figure in the analytic community - in fact, this is an understatement; he was one of the greats. Publishing many pathbreaking articles and books during his lifetime, he was a great teacher and always had a few tricks up his sleeve. Born in Oberlin, Ohio in 1940, he set out at an early age to learn a range of technical concepts. The adept Lewis was known to have constructed his own radio out of items found around the family home when he was only seven or eight years old. A stand-out pupil, he went on to study philosophy at university. He even went to England, where a leading academic who taught him wrote, more or less, 'This is a very promising student. He should work more to understand history, and work less on his own ideas, but he is very clever'. This was a female philosopher - it may have been G.E.M. Anscombe, who studied under Wittgenstein.
He then did his dissertation back in America, on the topic of Convention. His teacher was Quine, who was also a brilliant philosopher of course. How do we know which side (of the road) to drive on, and why do we pick one rather than the other? Why is the letter 'A' shaped like that, rather than like this: 'O'? These are questions about conventions, and the philosophical issues which arise here have perplexed many. Lewis had an intriguing new analysis of convention, involving the notion of a 'coordination problem'.
He also did work on counterfactual conditionals: if I don't do this, what will I do then? By analysing these, along with the contemporaneous Robert Stalnaker, using possible worlds, Lewis was able to come up with an original theory of the semantics of these interesting statements. If A were true C would be true, according to Lewis, is like saying that at the nearest A world, C is true. This has a lot to recommend it, and logicians have been discussing it ever since.
Liebniz also studied Possible Worlds
But this raised the question: what about these possible worlds? Here is where Lewis came into his own. Each world is a real one, just like ours. All of us have infinitely many counterparts like us, doing all the things we could have done. This radical idea enabled the daring but rigorous Lewis to give new theories of true and false statements, beliefs, possible and necessary outcomes, and of course those curious counterfactuals he started off with (described in the paragraph above). Most philosophers agree Lewis is mistaken, but they have had a hard time saying just how! As a result, huge literature has followed on from Lewis's writings on this topic (look for 'modal realism').
Everyone who knew Lewis said he was fun and interesting to be around, but sometimes his answers to a question could surprise people. Or his non-answers: one anectode says he took a long time to think before responsing, but the person asking didn't know this and kept asking him new questions. Someone else said 'Wait, he will answer!'. Sure enough, he did - very intelligently, no doubt.
David K. Lewis will be remembered for his contributions to metaphysics, epistemology, and language, to name just his main bailiwicks. He even wrote about ethics, and several other topics. But he also built a system. The metaphysics of David Lewis continues to exercise a powerful influence on contemporary philosophy, and though many of us would disagree with some of his theories - especially modal realism, which is impossible to believe in my opinion - we would certainly be all the poorer without them. Thank you David Lewis.