PZ Myers has an excellent post on extraterrestrial intelligence at "Pharyngula". I don't have anything to add to it beyond a small reaffirmation of something noted briefly in the comments discussion: from all the evidence we have, we have fairly good reason to doubt that, even on the questionable assumption that there are lots and lots of intelligent species, most of them would ever reach considerable technological advancement. An immense amount of our scientific development, for instance, is tied to size of our moon. Who knows how we would have developed scientifically and technologically if we never experienced total eclipses? It's possible, of course, that we would have done just fine without it (there were, after all, other things like comets); but when we're dealing with history, it's very hard to say how any of it would have happened had we had a different solar system. That's not as helpful a consideration as some of the ones Myers brings up (in part because it's something we know so much less about); but at the very least I think it is sometimes too easily assumed that the history of human intelligence shows some special predisposition to science, in a way that is parallel to (and as problematic as, or more problematic than) the assumption that the history of life shows some special predisposition to intelligence.
I should post something sometime on the "Plurality of Worlds" dispute (the eighteenth & nineteenth century version of this issue). Most of it is actually immensely boring (being of the silly "Creation has to manifest God's glory, so there must be intelligent life all over the place" sort), but my favorite Victorian, William Whewell, wrote a book on it (On the Plurality of Worlds). I've only read it once, and that only because Whewell wrote it; but I was pleasantly surprised, since Whewell occasionally makes the subject genuinely interesting. I really shouldn't have been surprised, because Whewell (who is the father of modern history and philosophy of science) tends to introduce all sorts of historical and methodological issues into most of the things he talks about. (For a sample you can see the page I promised on Whewell's analogy between the utilitarian principle in morals and the principle of least action in physics.) But that will have to wait for quite some time, given all the other things I have to do. If anyone's interested in the subject, there's a very brief overview, with some relevant 18th century texts here. (Link via Early Modern Resources.)
