Spent the morning reading this:

Edge.org link

A great question. Some of the respondents copped out in my opinion, avoiding the question by prattling on about the nature of proof and/or belief (semantic questions which may be useful to consider but that I found totally uninteresting in this case) but most of the respondents (scientists, philosophers, science writers/editors, etc.) came up with at least one interesting idea. Definitely worth the read. Lots of intelligent speculation in the areas of consciousness, evolution, and sub-atomic physics, with the widest spectrum of diverging opinions in those three areas as well.