John Locke,
An Essay Concerning Human Understanding…
“ For man knows that he himself exists. I think it is beyond question, that man has a clear idea of his own being ; he knows certainly he exists, and that he is something. He that can doubt whether he be anything or no, I speak not to ; no more than I would argue with pure nothing, or endeavour to convince nonentity that it were something. If any one pretends to be so sceptical as to deny his own existence, (for really to doubt of it is manifestly impossible,) let him for me enjoy his beloved happiness of being nothing, until hunger or some other pain convince him of the contrary. ”
