When I analyze the process that is expressed in the sentence 'I think', I find a whole series of daring assertions, the argumentative proof of which would be difficult, perhaps impossible: for instance, that it is I who thinks, 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'?

Beyond Good & Evil by Friedrich Nietzsche