caconway wrote:how can we be sure we exist?
You can't. The unfortunate fact of the matter is that all emperical evidence points to you being little more than a peculiar accumulation of quarks, or molecules, and that you, as you know yourself, will have a very brief and cosmically insignificant (though really, what isn't, in the great scheme of things) existence in this universe. If you're looking for substantiative affirmation of your physical or spiritual impermanence you might have difficulty finding it.
However, just because you might not really exist doesn't mean that you can't take some satisfaction in your experiences. Anyone who has played a video game knows that ficticious, temporary "realities" can be extremely entertaining, even if they mean absolutely nothing.
11:18 am, Greenwich Mean Time, December 21, 2012 AD.