Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

In every art, two things are chiefly to be considered: namely, the matter in which the art operates, and the form that is introduced into such matter. Thus, in building a house, the matter consists of stones and timber, while the form is their composition, because these are coordinated with one another in one figure and structure of a house. The craftsman does not make the matter, but presupposes it; he does, however, introduce the form. Since this is properly brought forth by the art, it is also principally intended by it, as its product. Logic is “a certain art whose office is to direct reason, lest it err in its manner of reasoning and knowing,” just as the building art directs the builder lest he err in building a house. Therefore Logic is called a rational art, not only because it is in reason as in a subject, like the other arts, but because the matter it directs consists of the very works of reason.

And since reason, in reasoning and passing judgment, proceeds by way of resolution—that is, by reducing things to their principles and discerning the proofs by which they are made manifest—it is the same thing for Logic to direct reason lest it err as to direct it to resolve rightly and duly. Hence Aristotle called the parts of Logic that teach one to render a certain judgment “analytic,” that is, resolutory, because they teach one to resolve rightly and without error. A right resolution arises both from due form and from the certainty of the matter. The matter consists of the things or objects that we wish to know rightly. The form, however, is the very manner or arrangement by which the objects known are connected, so that one may reason and know duly; for without connection no truth is conceived, nor does reasoning and inference proceed from one truth to another. Resolution on the side of form is said to pertain to prior resolution, whereas on the side of matter, according to due certainty and conditions, it pertains to posterior resolution, since in an art the consideration of an artificial form is prior to the consideration of matter.

From this, then, we take the division of the art of Logic and make two parts. In the first we shall treat all that pertains to the form of the art of Logic and to prior resolution, which the Philosopher treated in the books of Perihermenias and in the books of the Prior Analytics, and which is customarily taught to beginners in the Summulae. In the second part, however, we shall treat what pertains to logical matter, or to posterior resolution, especially in demonstration, to which Logic is principally ordered.

In this first part we compose a brief text for pupils to be instructed first, and then for those more advanced we dispute more difficult questions. In the second part, following the text of Porphyry and Aristotle set out in summary, we append more useful and weighty disputations.

Order of treatment. — Since Logic directs the manner of reasoning rightly, and there are three acts of reason by which one proceeds from one thing to another, as St. Thomas teaches, 1 Posterior Analytics, lecture 1.¹, no better order can be observed than to distribute the treatise on Logic according to these three operations. The first operation of our intellect is called simple apprehension, as when I understand a human being without affirming or denying anything of him. The second is composition or division, namely, when I know a thing in such a way that I attribute or deny something of it, as when I say that a human being is white, or deny that a human being is a stone. The third operation is reasoning, as when from some known truth I infer and gather another that is not so well known, as when from this truth, “a human being is rational,” I infer, “therefore he is teachable.” First, then, I apprehend the terms; next, I compose a proposition from them; finally, I form a reasoning from propositions.

Thus, in this first part we shall distribute the material into three books. The first concerns what pertains to the first operation, where we shall treat simple terms. The second concerns the second operation, where we shall treat discourse and proposition, and their properties. The third concerns the third, where we shall treat the manner of reasoning and of forming syllogisms and induction, and the other things that pertain to reasoning.

In the second part of Logic, however, we shall treat what pertains to the matter of such operations, chiefly insofar as it is ordered to forming a certain judgment from necessary truths, which is done through demonstration. Necessary truths, in turn, depend upon essential predicates, which are coordinated in the categories, and these upon the predicables, which express modes of predication, as will be explained more fully at the beginning of the second part of Logic. Nor is it unsuitable that simple terms and what pertains to the first operation be treated twice in Logic, because, as St. Thomas notes, 1 Perihermenias, lecture 1.², simple expressions are treated under one consideration in the Categories, namely, as signifying simple essences; under another in the book Perihermenias, namely, as parts of an enunciation; and under another in the books of the Prior Analytics, namely, as constituting the syllogistic order.

Finally, since one may proceed in reasoning in three ways to form a judgment—certainly, through demonstration; topically, through opinion; and erroneously, through sophisms—Aristotle, after treating demonstration and science in the books of the Posterior Analytics, treats what is opinion-based in the Topics and the sophistical syllogism in the Sophistical Refutations.

The necessity of this art is very great, both for the general reason that all arts are necessary so that a human being may be directed rightly and without error in his works, and especially because Logic directs the works of reason, on which all reasoning and inference depend, so that they may be right, free from error, and proceed in an orderly way. This is indeed very necessary for a human being who uses reason. But more on this in q. 1, proemium of the second part of Logic.