I recently posted this in reply to an atheist/skeptic who asked,"How do you know this?"

Unfortunately the answer is not as simple as the question. I'll tell you how you can begin to look for yourself though, if you're interested (which is a key ingredient). First you have to be willing to withdraw yourself from "authority" of any kind...including the scientific kind...and look at life and the world and yourself with your own eyes. Secondly, you have to determine whether anything spiritual is going on at all. Is consciousness confined to chemical reactions in the skull? Or is it non-local and can it access a common energy field that reaches far beyond the body? If you can determine this even in the slightest way, then the materialistic view comes crashing down like broken glass. From there you can take the third step which is to start taking seriously the witness testimony of those who have "been there" in consciousness. Near death experiences, past-life regression, between-life regression, OBE, paranormal testing with humans and animals, all sorts of one-off personal experiences, experiences in meditation and teachings of modern and ancient "gurus" and sages who have experienced things, etc. All of these gradually start to form a picture of something. And if you can combine this with some of your own meditation and ponderings and hopefully other experiences and learn to "hold paradox" as they say, never excluding anything, but leaving it all "on the table" as you go, until all contradictions fall away...then you'll have a nice chunk of it.