The thing with the question of oneness or non-oneness is that you can literally discuss it forever. You can go into the philosophy section of any library and you'll see people have been discussing it forever and will continue to do so.

If you want to believe in reincarnation, you have to believe that this life, what you're living through right now, is the afterlife. You're missing out on the afterlife you looked forward to in your last existence by worrying about your next life. This is what happens after you die. Take a look.

The thing with the question of oneness or non-oneness is that you can literally discuss it forever. You can go into the philosophy section of any library and you'll see people have been discussing it forever and will continue to do so.

No matter what we predict for our futures, we're always wrong anyway. The only sensible thing to do is to live this life as it is right now. Leave what happens after you die till after you die.

we ourselves are not something apart from our circumstances. What we are and where we are are one and the same.

I think the understanding of oneness and interconnectivity of the whole Universe is something we have innately, something we're born with. We are however very skillful at ignoring and pretending we don't have or know it.

Sanity and enlightenment...I've been reading a new book Dogen's Genjo Koan: Three Commentaries, and it contains a commentary on Genjo Koan by Shunryu Suzuki, the author who wrote Zen Mind, Beginners Mind. He doesn't mention sanity at all but I think that one possible definition of enlightenment would be a kind of profound sanity, where being insane is no longer an option.