Let the people suppose that knowledge means knowing things entirely; the philosopher must say to himself: When I analyze the process that is expressed in the sentence, "I think," I find a whole series of daring assertions that would be difficult, perhaps impossible, to prove; for example that it is I who think, that there must necessarily be something that thinks, that thinking is an activity and operation on the part of a being who is thought of as a cause, that there is an "ego," and, finally, that it is already determined what is to be designated by thinking--that I know what thinking is. For if I had not already decided within myself what it is, by what standard could I determine whether that which is just happening is not perhaps "willing" or "feeling"?

Nietzsche, Beyond Good and Evil, aphorism 16