Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

We must treat this third species of the method of knowing in the third book, to which it directly pertains, since it belongs to the third operation of the intellect. For the present, having distinguished the species of the method of knowing, it is enough to note three things:

FIRST is the definition of argumentation, which is “an expression in which, when one thing is given, another follows,” as when I say: “Peter is an animal; therefore, he is a body.” Hence argumentation must be composed of and consist in three things: the antecedent, as the inferring premise; the consequent, as what is inferred; and the sign of inference, which is the conjunction of antecedent and consequent that indicates inference or following.

SECOND, the species of argumentation are four: syllogism, enthymeme, induction, and example. They can be reduced to two, namely syllogism and induction, for an enthymeme is a kind of imperfect or truncated syllogism, and an example is a kind of part of induction. Argumentation is divided into these species insofar as it makes truth manifest, in which its quiddity consists. There are only two ways of making truth manifest: either by reasoning from principles known by the intellect, or by leading down to sensible things or singulars, since there is the origin of all our cognition. If it proceeds in the first way, it does so by syllogism, which consists of three propositions—the major, the minor, and the consequent—or of two, if it is an enthymeme, namely the antecedent and the consequent. Thus an enthymeme is an imperfect syllogism, because it sets down one proposition as antecedent, whereas a syllogism sets down two; the whole force of inference in a syllogism consists in the union of some things in a third, from which it infers their union with one another. If it proceeds in the second way, it is induction, which infers that something holds universally of all cases, not from connection with a third, but from a sufficient enumeration of several singulars. Example is reduced to this: from one singular it infers not something universal, but another similar thing.

THIRD, there are other divisions of argumentation besides these species. According to the sign of inference, one argumentation is inferential, namely, that made by the particle “therefore,” as: “Man is an animal; therefore, he is living.” Another is conditional, made by the particle “if,” as: “If the sun shines, it is day.” Another is causal, made by the particle “because,” as: “Because man is rational, he is capable of laughter.” These three differ in this: for the truth of a conditional, it is enough that the consequence be good, even if its extremes are false. For example, this conditional is true: “If man flies, he has wings,” although its extremes are false. For the truth of an inferential argument, the antecedent and consequent must be true. I say, for its truth; for as regards the goodness of the consequence, a truth can also follow from a falsehood, as we shall say in book 3¹. For the truth of a causal argument, the extremes must be true and the antecedent must be the cause of the consequent. For it is true that man is an animal and is capable of laughter, but false that he is capable of laughter because he is an animal.

According to quality, consequence is divided into good and bad. A good consequence is one in which the antecedent infers the consequent in such a way that the antecedent cannot be true while the consequent is false. A bad consequence is one in which the antecedent does not infer the consequent, although it seems or is represented as inferring it; because of that signification it is called a consequence, but a bad one, since it does not infer, and the antecedent is true while the consequent is false.

We shall speak of the rules of consequences for distinguishing good ones from bad ones in book 3, where the defects of consequence are to be treated expressly².