No, actually we do pretty much know…
Do you ever get tired of hearing people say “Well, we don’t know what happens after someone dies”? Or, “It’s not like anyone can come back to tell us what happens after we die, so we’re never going to know for sure”? And then they use that as a justification for believing in their pet afterlife hypothesis: heaven or reincarnation or spirits hanging around and sending us signs, or celebrity psychics who can talk to your deceased loved one for thousands of dollars.
The fact that we don’t — and can’t — have first-hand reports from the world beyond is no more proof of its existence than thoroughly debunked claims of near-death-experiences that “prove” that one exists.
We have reached a point when to believe that an individual’s consciousness survives the death of the brain requires ignoring the proliferation of evidence to the contrary.
If you are interested in the specific arguments, made in easily-understood terms by intelligent and articulate scientists, I highly recommend this video of the Intelligence2 debate “Death Is Not Final” featuring Cal Tech physicist Sean Carroll and Yale neurologist Steven Novella. Once you have watched it, you will at least have one or two better responses to “Well we don’t know…” than: