I used to be sure of my existence, because … how could I doubt it? After all, existing is one of those things you don’t need a certificate to prove it. You know it by getting up and being able to feel your heartbeat or the warmth of your partner’s breath. To know that you are alive, at least until a few years ago, the only thing you needed was to be sure of your own existence. No walking around there and asking the neighbor something like, “Hey, Mrs. Smith, I have my doubts. Could you tell me if I woke up alive today? ” I think something like that would seem absurd to most human beings. Nobody’s existence depends on the personal…