Everything that exists is evidence of causality, I am a necessary result of the chain of causality (starting with the "big bang"), hence, by observing everything, you can deduce that I exist. The only problem is that everything only exists in the mind, or consciousness, and consciousness does not physically exist; but at least in the context of consciousness, I exist. This also means that outside the context of consciousness, if I don't exist, you don't exist either. Within the context of your consciousness; I exist, as you within mine. There I proved to you that I exist.