"I think therefore I am".... This means that I know I exist but I dont know you exist... This then leads onto metaphysical skepticism. The whole universe and everyone and thing in it, except for myself does not exist, other than within my own thoughts. Keeping this simple. If we consider the big bang, the creation of the universe seems to be impossible as to why it happened in the first place. Not how, but why. It would be far simpler to conclude it didnt happen and therefore does not exist. This supports: I think therefore I am but you may not exist. "Cogito ergo sum,.. however, I dont know you exist, prove to me you exist."