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