Metaphysics by Aristotle
Book IV, 1005b-1006b
Natural philosophy is a kind of Wisdom, but not the primary kind. As for the attempts of some of those who discuss how the truth should be received, they are due to lack of training in logic; for they should understand these things before they approach their task, and not investigate while they are still learning.Clearly then it is the function of the philosopher, i.e. the student of the whole of reality in its essential nature, to investigate also the principles of syllogistic reasoning. And it is proper for him who best understands each class of subject to be able to state the most certain principles of that subject; so that he who understands the modes of Being qua Being should be able to state the most certain principles of all things.Now this person is the philosopher, and the most certain principle of all is that about which one cannot be mistaken; for such a principle must be both the most familiar (for it is about the unfamiliar that errors are always made), and not based on hypothesis.For the principle which the student of any form of Being must grasp is no hypothesis; and that which a man must know if he knows anything he must bring with him to his task.Clearly, then, it is a principle of this kind that is the most certain of all principles. Let us next state what this principle is.”It is impossible for the same attribute at once to belong and not to belong to the same thing and in the same relation”; and we must add any further qualifications that may be necessary to meet logical objections. This is the most certain of all principles, since it possesses the required definition; for it is impossible for anyone to suppose that the same thing is and is not, as some imagine that Heraclitus says—for what a man says does not necessarily represent what he believes. And if it is impossible for contrary attributes to belong at the same time to the same subject (the usual qualifications must be added to this premise also), and an opinion which contradicts another is contrary to it, then clearly it is impossible for the same man to suppose at the same time that the same thing is and is not; for the man who made this error would entertain two contrary opinions at the same time.Hence all men who are demonstrating anything refer back to this as an ultimate belief; for it is by nature the starting-point of all the other axioms as well.
There are some, however, as we have said, who both state themselves that the same thing can be and not be, and say that it is possible to hold this view. Many even of the physicists adopt this theory. But we have just assumed that it is impossible at once to be and not to be, and by this means we have proved that this is the most certain of all principles.Some, indeed, demand to have the law proved, but this is because they lack education; for it shows lack of education not to know of what we should require proof, and of what we should not. For it is quite impossible that everything should have a proof; the process would go on to infinity, so that even so there would be no proof. If on the other hand there are some things of which no proof need be sought, they cannot say what principle they think to be more self-evident. Even in the case of this law, however, we can demonstrate the impossibility by refutation, if only our opponent makes some statement. If he makes none, it is absurd to seek for an argument against one who has no arguments of his own about anything, in so far as he has none; for such a person, in so far as he is such, is really no better than a vegetable. And I say that proof by refutation differs from simple proof in that he who attempts to prove might seem to beg the fundamental question, whereas if the discussion is provoked thus by someone else, refutation and not proof will result.The starting-point for all such discussions is not the claim that he should state that something is or is not so (because this might be supposed to be a begging of the question), but that he should say something significant both to himself and to another (this is essential if any argument is to follow; for otherwise such a person cannot reason either with himself or with another);and if this is granted, demonstration will be possible, for there will be something already defined. But the person responsible is not he who demonstrates but he who acquiesces; for though he disowns reason he acquiesces to reason. Moreover, he who makes such an admission as this has admitted the truth of something apart from demonstration [so that not everything will be “so and not so”].Thus in the first place it is obvious that this at any rate is true: that the term “to be” or “not to be” has a definite meaning; so that not everything can be “so and not so.” Again, if “man” has one meaning, let this be “two-footed animal.”By “has one meaning” I mean this: if X means “man,” then if anything is a man, its humanity will consist in being X. And it makes no difference even if it be said that “man” has several meanings, provided that they are limited in number; for one could assign a different name to each formula.For instance, it might be said that “man” has not one meaning but several, one of which has the formula “two-footed animal,” and there might be many other formulae as well, if they were limited in number; for a particular name could be assigned to each for formula.If on the other hand it be said that “man” has an infinite number of meanings, obviously there can be no discourse; for not to have one meaning is to have no meaning, and if words have no meaning there is an end of discourse with others, and even, strictly speaking, with oneself; because it is impossible to think of anything if we do not think of one thing; and even if this were possible, one name might be assigned to that of which we think.Now let this name, as we said at the beginning, have a meaning; and let it have one meaning. Now it is impossible that “being man” should have the same meaning as “not being man,” that is, if “man” is not merely predicable of one subject but has one meaning(for we do not identify “having one meaning” with “being predicable of one subject,” since in this case “cultured” and “white” and “man” would have one meaning, and so all things would be one; for they would all have the same meaning). And it will be impossible for the same thing to be and not to be, except by equivocation, as e.g. one whom we call “man” others might call “not-man”;but the problem is whether the same thing can at once be and not be “man,” not in name , but in fact . If “man” and “not-man” have not different meanings, clearly “not being a man” will mean nothing different from “being a man”; and so “being a man” will be “not being a man”; they will be one.For “to be one” means, as in the case of “garment” and “coat,” that the formula is one. And if “being man” and “being not-man” are to be one, they will have the same meaning; but it has been proved above that they have different meanings. If then anything can be truly said to be “man,” it must be “two-footed animal”; for this is what “man” was intended to mean. And if this is necessarily so, it is impossible that at the same time the same thing should not be “two-footed animal.” For “to be necessarily so” means this: that it is impossible not to be so. Thus it cannot be true to say at the same time that the same thing is and is not man. And the same argument holds also in the case of not being man…