"I've long suspected that the rise of spacefaring intelligent life is so unlikely that we will never encounter another within our causal lightcone. In a universe of infinite size (as ours seems to be) the weak anthropic principle has an infinitely powerful ability to explain our existence, no matter how unlikely. As a great man once said, there are the known unknowns, and the unknown unknowns. We know that we don't yet know some basic things, such as how likely habitable planets are (to date we've only found hunks of rock in zones that might conceivably be habitable), how unlikely the eukaryotic mutations were, how unlikely intelligent tool-users are to evolve, how unlikely the development of a scientific Western Civ was (remember, most civs were still Neolithic even a few hundreds years ago), and even how likely free capitalist societies are (what if the Communists or Nazis had won out in this world?). But there are probably even more things we still don't know about the chain of coincidences that led us here. The Drake equation probably ends up with quite a few more variables than is generally appreciated."