EPISTEMOLOGY Psychology is interested in the cause of our thinking, mathematics in the proof of our thoughts Cause and proof are quite different things Without an appropriate ration of phosphorus in his brain, no doubt, Pythagoras would have been unable to prove his famous theorem; but that does not mean that a statement of the phosphorus content of his brain should occur as a line in the proof If humans have evolved, no doubt there has been evolution in human consciousness; so if mathematics was a matter of sensations and ideas, we would need to warn astronomers against drawing conclusions about events in the distant past Frege brings out the absurdity of this position in an ironic passage: You reckon that  ¼ 4; but the idea of number has a history, an evolution It may be doubted whether it had yet progressed so far How you know that in that distant past that proposition already existed? Might not the creatures then alive have held the proposition  ¼ 5? Perhaps it was only later that natural selection, in the struggle for existence, evolved the proposition  ¼ 4, and perhaps that in its turn is destined to develop into  ¼ (FA, pp vi–vii) Throughout his life, Frege continued to maintain a sharp distinction between logic and psychology In his late essay ‘Thoughts’ he warned against the ambiguity inherent in the statement that logic deals with the laws of thought If, by ‘laws of thought’, we mean psychological laws that relate mental events to their causes, then they are not laws of logic because they would make no distinction between true and false thoughts, since error and superstition have causes no less than sound belief Logical laws are ‘laws of thought’ only in the same sense as moral laws are laws of behaviour Actual thinking does not always obey the laws of logic any more than actual behaviour always obeys the moral law However, in his late ‘Thoughts’ Frege ventures into epistemology in a manner that tends to blur the distinctions he had so resolutely defended He inquires about the sense, or mode of presentation, of the first-person pronoun ‘I’, which he treats as a proper name that has its user as its reference Everyone, Frege says, ‘is presented to himself in a special and primary way, in which he is presented to no one else’ Suppose that Horatio has the thought that he has been wounded Only he can grasp the sense of that thought, since it is only to himself that he is presented in this special way He cannot communicate a thought he alone can grasp Therefore, if he now says ‘I have been wounded’ he must use ‘I’ in a sense which can be grasped by others, 157