… [I feel that] it is necessary to know what doubt is, and what thought is, [what existence is], before we can be fully persuaded of this reasoning — I doubt, therefore I am — or what is the same — I think, therefore I am.
(René Descartes, Inquisitio Veritatis per Lumen Naturale).[1]
“There are still harmless observers of themselves who believe that there are "immediate certainties," for example, "I think," or that superstition of Schopenhauer's, "I will," just as if perception here was able to seize upon its object pure and naked, as "thing in itself," and as if there was no falsification either on the part of the subject or on the part of the object. However, the fact is that "immediate certainty," just as much as "absolute cognition" and "thing in itself," contains within itself a contradictio in adjecto [contradiction in terms]. I'll repeat it a hundred times: people should finally free themselves of the seduction of words! Let folk believe that knowing is knowing all of something. The philosopher must say to himself, "When I dismantle the process which is expressed in the sentence 'I think,' I come upon a series of daring assertions whose grounding is difficult, perhaps impossible - for example, that I am the one who thinks, that there must be some general something that thinks, that thinking is an action and effect of a being which is to be thought of as a cause, that there is an 'I', and finally that it is already established what we mean by thinking - that I know what thinking is. For if I had not yet decided these questions in myself, how could I assess that what just happened might not perhaps be 'willing' or 'feeling'?" In short, this "I think" presupposes that I compare my immediate condition with other conditions which I know in myself in order to establish what it is. Because of this referring back to other forms of "knowing," it certainly does not have any immediate "certainty" for me. Thus, instead of that "immediate certainty," which the people may believe in the case under discussion, the philosopher encounters a series of metaphysical questions, really essential problems of intellectual knowledge, as follows: "Where do I acquire the idea of thinking? Why do I believe in cause and effect? What gives me the right to speak of an 'I,' and indeed of an 'I' as a cause, finally even of an 'I' as the cause of thinking?" Anyone who dares to answer those metaphysical questions right away with an appeal to some kind of intuitive cognition, as does the man who says "I think and know that at least this is true, real, and certain"- such a person nowadays will be met by a philosopher with a smile and two question marks. "My dear sir," the philosopher will perhaps give him to understand, "it is unlikely that you are not mistaken but why such absolute truth?"” (Friedrich Nietzsche - Beyond Good and Evil, Part I - Aphorism # 16) [2]
"So far as the superstitions of the logicians are concerned, I will never tire of emphasizing over and over again a small brief fact which these superstitious types are unhappy to concede - namely, that a thought comes when "it" wants to and not when "I" wish, so that it's a falsification of the facts to say that the subject "I" is the condition of the predicate "think." It thinks: but that this "it" is precisely that old, celebrated "I" is, to put it mildly, only an assumption, an assertion, in no way an "immediate certainty." After all, we've already done too much with this "it thinks": this "it" already contains an interpretation of the event and is not part of the process itself. Following grammatical habits we conclude here as follows: "Thinking is an activity. To every activity belongs someone who does the action, therefore -." With something close to this same pattern, the older atomists, in addition to the "force" which created effects, also looked for that clump of matter where the force was located, out of which it worked - the atom. Stronger heads finally learned how to cope without this "remnant of earth," and perhaps one day people, including even the logicians, will also grow accustomed to cope without that little "it" (to which the honourable old "I" has reduced itself)." (Friedrich Nietzsche - Beyond Good and Evil, Part I - Aphorism #17) [3]
[1] trans. Hallam: ... [S]entio, oportere, ut quid dubitatio, quid cogitatio, quid exsistentia sit antè sciamus, quàm de veritate hujus ratiocinii: dubito, ergo sum, vel, quod idem est, cogito, ergo sum: plane simus persuasi.
[2] trans. nietzsche.holtof.com
[3] trans. nietzsche.holtof.com