Thomas Aquinas describes the nature of the art of logic as analogous to manual arts in his introduction to his Commentary on Aristotle’s Posterior Analytics.
As Aristotle says in the beginning of the Metaphysics,the human race lives by art and reasonings. In this, the Philosopher seems to touch on a certain property by which man differs from the other animals which are prompted to their acts by a natural impulse, while man is directed in his actions by a judgment of reason. And this is the reason why there are various arts devoted to the easy and orderly performance of human acts. For an art seems to be nothing other than a certain ordering of reason, by which human acts reach their due end through appropriate means. Now, reason is not only able to direct the acts of the lower powers of man, but is also directive of its own act, for this is proper to the intellective part of man that it is able to reflect upon itself; for the intellect knows itself, and similarly reason is able to reason about its own acts. If, therefore, the art of building or carpentry, through which man is able to perform manual acts in an easy and orderly manner, arose from reasoning about manual acts, then by the same reason a certain art is needed to direct reason’s own act of reasoning, so that by it a man might proceed in an orderly and easy manner and without error.
And this art is logic, that is, the science of reason. It concerns reason not only from the fact that it proceeds according to reason, which is common to all arts, but also from the fact that it is concerned with the very act of reasoning as its proper matter.
Therefore, it seems to be the art of the arts, because it directs us in the act of reasoning from which all arts proceed.
The art of logic, then, has the goal of rationally ordering the acts of reason, as the art of carpentry orders the acts of fashioning wood so as to produce wooden products well: good chairs, tables, or houses. Logic uses well-ordered acts of reason ultimately to produce new intellectual knowledge from pre-existing sense knowledge, which knowledge it orders and organizes into the various practical arts (liberal or servile) and sciences, speculative or theoretical (biology, physics, metaphysics) and practical (ethics, politics).
The Acts of the Intellect and the Parts of Logic
The art of logic is itself ordered and organized according to the various acts which the faculty or power of reason exercises, and so Thomas then immediately lists these parts of logic by these acts:
It is necessary, therefore, to take the parts of logic according to the diversity among the acts of reason. Now, there are three acts of reason, the first two of which belong to reason as a certain understanding.
One action of the intellect is the understanding of indivisible or uncomplex things, according to which it conceives what a thing is. This operation is called by some the “informing of the intellect,” or “representing by means of the intellect.” To this operation of the reason is ordered the teaching which Aristotle hands down in the book of Categories.
The second operation of the intellect is composition or division, in which there is present the true or the false. And this act of reason is the subject of the teaching which Aristotle hands down in the book entitled On Interpretation.
The third act of reason is concerned with that which is proper to reason, namely, to advance from one thing to another so that through that which is known one comes to a knowledge of the unknown. And this act is considered in the remaining books of logic.
Traditionally, these acts of reason, and the corresponding parts of logic concerned with ordering each to perform its acts well, are as follows:
- Simple Apprehension grasps what a thing is, its nature or essence (quiddity or ‘whatness’ from quid, Latin for ‘what’) and expresses the concepts it forms in words or terms. The mind orders this act by forming proper definitions of terms to aid in knowing what exactly things are.
- Judgment either composes or affirms one thing with another, or divides, separates, and denies one thing from another. Propositions express these judgments about things positively or negatively, universally or partially (as particular). Truth or falsity is first found in judgments, and there are, as we will see below, interesting relationships between them according to whether they affirm or deny universally or partially.
- Reasoning (properly so called) draws conclusions or arrives at new knowledge from premises expressing prior knowledge through syllogisms or arguments. Based on a term linking or mediating the subject of a conclusion with its predicate in the premises known beforehand, and the form in which these subject, middle, and predicate terms are arranged, reason draws true and necessary conclusions.
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Updated January 18, 2025
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