Commentary on the Posterior Analytics of Aristotle St. Thomas Aquinas (translated by Fabian R. Larcher, O.P.) Foreword (of St. Thomas Aquinas) As the Philosopher says in Metaphysics I (980b26), “the human Consequently one should view the parts of logic according to the diversity among the acts of reason. Now there are three acts of the reason, the first two of which belong to reason regarded as an intellect. One action of the intellect is the race lives by art and reasonings.” In this statement the Philosopher understanding of indivisible or uncomplex things and according to this seems to touch upon that property whereby man differs from the other action it conceives what a thing is. And this operation is called by some animals. For the other animals are prompted to their acts by a natural the informing of the intellect, or representing by means of the intellect. impulse, but man is directed in his actions by judgment of reason. And To this operation of the reason is ordained the doctrine which Aristotle this is the reason why there are various arts devoted to the ready and hands down in the book of Predicaments, [i.e., Categories]. The second orderly performance of human acts. For an art seems to be nothing more operation of the intellect is its act of combining or dividing, in which the than a definite and fixed procedure established by reason, whereby true or the false are for the first time present. And this act of reason is the human acts reach their due end through appropriate means. subject of the doctrine which Aristotle hands down in the book entitled Now reason is not only able to direct the acts of the lower powers but is also director of its own act: for what is peculiar to the intellective part of man is its ability to reflect upon itself. For the intellect knows itself. In like manner reason is able to reason about its own act. Therefore jus as the art of building or carpentering, through which man On Interpretation. But the third act of the reason is concerned with that which is peculiar to reason, namely, to advance from one thing to another in such a way that through that which s known a man comes to a knowledge of the unknown. And this act is concerned in the remaining books of logic. is enabled to perform manual acts in an easy and orderly manner, arose It should be noted that the acts of reason are in a certain sense from the fact that reason reasoned about manual acts, so in like manner not unlike the acts of nature: hence so far as it can, art imitates nature. an art is need to direct the act of reasoning, so that by it a man when Now in the acts of nature we observe a threefold diversity. For in some of performing the act of reasoning might proceed in an orderly and easy them nature acts from necessity, i.e., in such a way that it cannot fail; in manner and without error. And this art is logic, i.e., the science of reason. others, nature acts so as to succeed for the most part, although now and And it concerns reason not only because it is according to reason, for that then it fails in its act. Hence in this latter case there must be a twofold act: is common to all arts, but also because it is concerned with the very act of one which succeeds in the majority of cases, as when from seed is reasoning as with its proper matter. Therefore it seems to be the art or generated a perfect animal; the other when nature fails in regard to what the arts, because it directs us in the act of reasoning, from which all arts is appropriate to it, as when from seed something monstrous is proceed. generated owing to a defect in some principle. These three are found also in the acts of the reason. For there is accordingly as one approaches more or less to complete certitude. For one process of reason which induces necessity, where it is not possible to although science is not obtained by this process of reason, nevertheless fall short of the truth; and but such a process of reasoning the certainty of belief or opinion is sometimes achieved (on account of the probability of science is acquired. Again, there is a process of reason in which the propositions one starts with), because reason leans completely to something true in most cases is concluded but without producing one side of a contradiction but with fear concerning the other side. The necessity. But the third process of reason is that in which reason fails to Topics or dialectics is devoted to this. For the dialectical syllogism which reach a truth because some principle which should have been observed Aristotle treats in the book of Topics proceeds from premises which are in reasoning was defective. provable. Now the part of logic which is devoted to the first process is At times, however, belief or opinion is not altogether achieved, called the judicative part, because it leads to judgments possessed of the but suspicion is, because reason does not lean to one side of a certitude of science. And because a certain and sure judgment touching contradiction unreservedly, although it is inclined more to one side than effects cannot be obtained except by analyzing them into their first to the other. To this the Rhetoric is devoted. At other times a mere fancy principles, this part is called analytical, i.e., resolvent. Furthermore, the inclines one to one side of a contradiction because of some certitude obtained by such an analysis of a judgment is derived either representation, much as a man turns in disgust from certain food if it is from the mere form of the syllogism—and to this is ordained the book of described to him in terms of something disgusting. And to this is the Prior Analytics which treats of the syllogism as such—or from the ordained the Poetics. For the poet’s task is to lead us to something matter along with the form, because the propositions employed are per virtuous by some excellent description. And all these pertain to the se and necessary [cf. infra, Lectures 10, 13]—and to this is ordained the philosophy of the reason, for it belongs to reason to pass from one thing book of the Posterior Analytics which is concerned with the to another. demonstrative syllogism. To the second process of reason another part of logic called investigative is devoted. For investigation is not always accompanied by certitude. Hence in order to have certitude a judgment must be formed, bearing on that which has been investigated. But just as in the works of nature which succeed in the majority of cases certain levels are achieved—because the stronger the power of nature the more rarely does it fail to achieve its effect—so too in that process of reason which is not accompanied by complete certitude certain levels are found The third process of reasoning is served by that part of logic which is called sophistry, which Aristotle treats in the book On Sophistical Refutations.