From Nietzsche: “There are still harmless self-observers who believe that there are ‘immediate certainties’; for instance, ‘I think,’ …] as though cognition here got hold of its object purely and simply as ‘the thing in itself,’ without any falsification taking place either on the part of the subject or the object. I would repeat it, however, a hundred times, that ‘immediate certainty,’ as well as ‘absolute knowledge’ and the ‘thing in itself,’ involve a contradictio in adjecto; we really ought to free ourselves from the misleading significance of words! …] 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, the argumentative proof of which would be difficult, perhaps impossible: for instance, 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”? In short, the assertion “I think,” assumes that I compare my state at the present moment with the other states of myself which I know, in order to determine what it is; on account of this retrospective connection with further “knowledge,” it has, at any rate, no immediate certainty for me.’ — In place of the ‘immediate certainty’ in which the people may believe in the special case, the philosopher thus finds a series of metaphysical questions presented to him, veritable conscience questions of the intellect, to wit: ‘Whence did I get the notion of “thinking”? Why do I believe in cause and effect? What gives me the right to speak of an “ego,” and even of an “ego” as cause, and finally of an “ego” as cause of thought?’ He who ventures to answer these metaphysical questions at once by an appeal to a sort of intuitive perception, like the person who says, ‘I think, and know that this, at least, is true, actual, and certain’ — will encounter a smile and two notes of interrogation in a philosopher nowadays. 'Sir,” the philosopher will perhaps give him to understand, ‘it is improbable that you are not mistaken, but why should it be the truth?’"
interesting observation, never thought of it that way. Aquinas’s proofs of God are not that convincing to me, except from his argument from contingency.