Hume's A Treatise of Human Nature (I, iv, 6): Personal Identity
Abstract
We are every moment intimately conscious of what we call our self; we feel its existence and its continuing to exist, and are certain - more even than any demonstration could make us - both of its perfect identity and of its simplicity. The strongest sensations and most violent emotions, instead of distracting us from this view ·of our self·, only focus it all the more intensely, making us think about how these sensations and emotions affect our self by bringing it pain or pleasure. To offer further evidence of the existence of one’s self would make it less evident, not more, because no fact we could use as evidence is as intimately present to our consciousness as is the existence of our self. If we doubt the latter, we can’t be certain of anything