P. F. Strawson - Introduction to Logical Theory

1. WHEN a man says or writes something, there are many different ways in which his performance may be judged. Among other things, we may question his truthfulness or criticize his style, we may assess the morality of what he says, or we may appraise its logic; though not all these types of assessment are appropriate to all kinds of utterance. The words logical and illogical are themselves among the words of logical appraisal. If you call a discourse logical, you are in some degree commending it. If you call it illogical, you are, so far, condemning it. Words and phrases which go with logical are consistent, cogent, valid, it follows; words and phrases which go with illogical, are inconsistent, self-contradictory, invalid, a non sequitur. Part of our problem is to see what sort of appraisal these words are used for to what kind of standards we appeal in using them. It is easy to see that these are not moral or aesthetic standards; that logical criticism is not, say, a kind of literary criticism. A slightly more difficult distinction is that between the criticism we offer when we declare a man's remarks to be untrue and the criticism we offer when we declare them to be inconsistent. In the first case we criticize his remarks on the ground that they fail to square with the facts; in the second case we criticize them on the ground that they fail to square with one another. The charge of untruth refers beyond the words and sentences the man uses to that in the world about which he talks. We deny his assertion, and, in doing so, make a counter-assertion of our own about the subject of his discourse. We contradict him. But the charge of inconsistency does not in this way refer to anything outside the statements that the man makes. We simply consider the way his statements hang together. Just from considering the sentences themselves, as they are used, we can, perhaps, see that not all the statements he makes can be true together. It is not that we contradict him, and in doing so, make a counter-assertion about the subject of his remarks;we assert that he has contradicted himself, and, in doing this, we make no appeal to the facts and express no opinion about them. It is this kind of internal criticism that is appraisal of the logic of a piece of discourse.

2. Words of logical appraisal have connected meanings. To be clear about the meaning of one such word is to be clear about the meanings of the others. For example, in a proof or argument, one statement (the conclusion) is said to follow logically from, or to be logically implied by, others (the premises), if the argument is valid; and an argument is valid only if it would be inconsistent (or self-contradictory) to assert the premises while denying the conclusion; or, in other words, only if the truth of the premises is inconsistent with the falsity of the conclusion. A deductive argument is a sort of threat, which takes the form: if you accept these premises as true, then you must accept this conclusion as true as well, on pain of self-contradiction. From among the various concepts of logical appraisal, I shall select this notion of inconsistency or self-contradiction for detailed discussion. Other choices could have been made, but there are reasons, which will emerge as we go on, for making this choice.

3. What is inconsistency? It is better to approach this question indirectly, by asking a series of others, One might ask first: Why bother to avoid inconsistency? What is wrong with contradicting yourself? There is nothing morally wrong about it. It may not even be entirely pointless. Suppose a man sets out to walk to a certain place; but, when he gets half-way there, turns round and comes back again. This may not be pointless. He may, after all, have wanted only exercise. But, from the point of view of a change of position, it is as if he had never set out. And so a man who contradicts himself may have succeeded in exercising his vocal chords. But from the point of view of imparting information, of communicating facts (or falsehoods) it is as if he had never opened his mouth. He utters words, but does not say anything. Or he might be compared with a man who makes as if to give something away and then takes it back again. He arouses expectations which he does notfulfil; and this may have been his purpose. Similarly, it may have been the purpose of a man who contradicts himself just to create puzzlement. The point is that the