The act or process of reasoning to a necessary conclusion using demonstrative syllogisms produces science in the strict sense, acts of knowledge and the habit or disposition in the soul to entertain such knowledge. There are, then, various sciences, each as an organized set or ordered collection of such demonstrative syllogisms on various subjects. The habit (or intellectual virtue) of science may be in the soul of a person either as doctrine, when possessed by the teacher who assists another in acquiring scientific reasoning, or as discipline, when possessed by a student. Science in the strict sense means the knowledge we possess (whether actually entertained or habitually held) of the cause on which a thing depends, that it is the cause, and that the thing cannot be other than it is. Thus, “science is certain knowledge through causes.”[1]
There are two kinds of demonstration that produce science either to a perfect or less than perfect degree according to the extent that knowledge of a thing’s causes leads to knowledge of that thing. The more perfect demonstration of the reasoned fact (demonstratio propter quid) is the knowledge that some fact is necessarily true through the causes of that fact. The less perfect demonstration of the fact (demonstratio quia) is simply knowledge that the fact is necessarily true, but this does not come through the causes of that fact, but usually through its effects. It tells us what is true, but not why it is true.
[W]e may infer the cause from the effect (on the basis of necessary causal principles) rather than the effect from the cause. Aquinas calls demonstrations of this sort factual demonstrations (demonstrationes quia) because they establish that something is the case without providing a theoretically deep explanation of it of the sort metaphysically prior fact would provide. By contrast, he calls demonstrations the premises of which give the cause or explanation for the conclusion explanatory demonstrations (demonstrationes propter quid). Paradigmatic scientia requires explanatory demonstration, but merely factual demonstration gives us scientia of a sort.[2]
In demonstrations propter quid, the syllogism identifies the proper cause for identifying the predicate with the subject in the conclusion, i.e., when the middle term is the definition of the subject, and through this middle term, we prove that property belongs to the subject of the conclusion. For example,
Every rational animal is capable of humor.
Every man is a rational animal.
Therefore, every man is capable of humor.
Being a rational animal is the cause and reason that every human is capable of humor; a thing must have reason in order to find something funny or to tell a joke, a fact not lost on Commander Data in the tv series, Star Trek: The Next Generation. Demonstration in the strictest sense is demonstration propter quid since it provides both knowledge of the necessity of the fact and of the reason why the fact is true. Human persons, as rational animals (or rational fictive machines) must necessarily be capable of humor precisely because humor depends on being rational.
Demonstration in the weaker sense is demonstration quia, since it only provides knowledge that the fact must be true. Demonstration quia starts from the effect and deduces the cause. Thomas Aquinas in ST I, q. 2, a, 2, for instance, writes that one comes to know that God exists through a demonstration quia; it is from seeing that the world, or various aspects of it, must have a cause, that one can deduce the existence of its intelligent and all-powerful Creator.
In Posterior Analytics, Bk. I, c. 13, Aristotle explains exactly how demonstration quia works with another example taken from astronomy.
Quia demonstration begins from what is more known to us, propter quid from the cause. For example, suppose that we want to understand why the planets, as opposed to all the other stars, do not twinkle. We notice that when lights are relatively close, they do not twinkle, but when they are far away, they do. Therefore we make the following syllogism:
Every light that does not twinkle is relatively near.
Every planet is light that does not twinkle.
Therefore, every planet is relatively near.Aristotle points out that this syllogism, while valid, reverses the order of cause and effect. Nearness is the cause of not-twinkling, not vice-versa; and yet nearness is in the conclusion, while not-twinkling is in the premises. Such a syllogism, he points out, does prove the fact of nearness, but it does not prove it from its true cause, which is nearness.[3]
As Aristotle writes,
This syllogism, then, proves not the reasoned fact but only the fact; since [planets] are not near because they do not twinkle, but, they do not twinkle because they are near.[4]
Demonstrations propter quid are often called a priori reasoning in modern parlance; demonstrations quia equate to a posteriori reasoning.
Valid Logical Relations Reflect Reality
Throughout and behind all of Aquinas’s doctrine and use of Aristotle’s logic is the conviction that how our minds operate according to the three acts of the intellect, and the concepts, judgments, and conclusions we produce in the art and science of logic are able to uncover the truth of things precisely because the mental acts and products reflect the metaphysical structure of reality. As noted before, naturally occurring substances and accidents are instances of real, objective essential natures, and they provide the objective content for universal categorical propositions. The terms we define name genuine natural kinds in the first act of the intellect and refer to real natures. The real definitions we form “explicate these natures by identifying a kind’s genus and specifying differentia (which are also real natures).”[5] This moderate or inherent realist metaphysics and epistemology underpin Aristotle and Aquinas’ logic and explain and guarantee that logical inferences drawn from an immediate contact with the actuality of reality in sense perception produce a true understanding of real causes of the conclusions of the syllogisms or arguments in the sciences (scientiae) broadly considered.
[1] John A. Oesterle, Logic: The Art of Defining and Reasoning (Prentice-Hall 1952), p. 177.
[2] Scott MacDonald, “Theory of Knowledge” in The Cambridge Companion to Aquinas, p. 175.
[3] Anthony Andres, Introductory Logic (Arts of Liberty Project 2017), p. 108.
[5] MacDonald, art. cit., p. 169.
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