On the middle term in demonstration
From the articles, however, in which a methodical inquiry is instituted into the real definition of anything, we must distinguish and otherwise explain those in which St. Thomas solves the question, whether a thing is of such a nature, and often he solves as one question the composite: For what purpose is it of such a nature? Examples of such are: when he asks whether the human soul is incorruptible (that is, whether and for what purpose it is incorruptible); whether man is free, whether faith is most certain, whether it belongs to God alone to create, whether and for what purpose Christ's passion was the cause of our salvation by way of merit, and other similar questions. In these cases the solution of the question for what purpose, refers to a true and indeed a priori demonstration, nor does it mean one derived from common but from proper principles. Hence in these last-mentioned articles that are strictly demonstrative, whether they are deduced from reason alone or from faith and reason, a special inquiry must be made into the middle term of the demonstration, which is, as it were, the golden key of the article.
The title of the article gives the two terms of the conclusion, namely, the minor and the major; the middle term must be assigned by which the other two can be united in a scientific conclusion, and this term assigns "why a thing is and cannot be otherwise than it is." It is the very Aristotelian definition of scientific knowledge .(32)
Sometimes,_however, in the composition of the body of these articles, St. Thomas begins with the major and through the minor descends to the conclusion, so that the argument is easily presented in scholastic form as to make it clear what is the middle term in the demonstration. Thus, in the question, "Whether the human soul is incorruptible," (33) the argument may be condensed into the following syllogism: Every simple and subsistent form is absolutely incorruptible. But the human soul is a simple and subsistent form. Therefore the human soul is incorruptible. Likewise, in the question, "Whether it belongs to God alone to create," (34) the argument may be reduced to this syllogism: The most universal effects must be reduced to the most universal and first cause, and that is God. Now being itself, which is absolutely produced in creation, is the most universal of effects. Therefore to produce being absolutely, not as this or that being, or to create, belongs to God alone.
Often, too, St. Thomas begins with the minor, the subject of which is already given in the title and will appear again as the subject of the conclusion. Thus by the minor he ascends from the subject of the title to the middle term in the demonstration. Afterward he enunciates the major, its subject being the same middle term, its predicate being the major term of the title, which in the conclusion must be joined to the minor term. Thus often the process of proof in the article is by the ascent from the minor to the middle term in the demonstration, and by the descent from the major to the conclusion. We have an example of this in the question: "Whether any created good constitutes man's happiness." (35) St. Thomas replies by enunciating first the minor: Happiness is the perfect good, completely lulling the rational appetite which is specified by universal good; now the perfect good, which completely lulls the rational appetite that is specified by universal good, cannot be anything created or limited; therefore man's happiness cannot consist in any created good.
If we wish to present the argument in syllogistic form, the major must be enunciated first. In the generality of cases, by retaining the very propositions of St. Thomas, the argument can be reduced to scholastic form. It is better, however, to keep to the Doctor's own terms than to change them so as to follow an excessive logical formalism. Finally, the major or minor must be defended against the attacks made upon it by the opponents of St. Thomas.
In the explanation of the body of the article the middle term of the demonstration must be the subject of diligent inquiry, or, if there are several subordinate middle terms, evidently we must concentrate our attention upon the principal one. The reason is that, as St. Thomas often remarks "the conclusions are known materially; but the middle terms in the demonstration are the formal cause of our knowledge, and by these the conclusions are known." (36) Thus it is known formally for what purpose a thing is of such a nature, for instance, why man is free. It is because he has knowledge of universal good that his attitude toward some particular good is one of dominating indifference. Or again, why man is a social being; this is because of the requirements of his specific act, which are to know those things which he needs to know. Because of his very limited intelligence he needs the assistance of others.
Thus there is only one formal or proximate middle term, which is the definition of the thing as to its essence, from which the first property is to be deduced, and from this first property the one subordinate to this, and so on in ascending order. Nevertheless, anything that has already been demonstrated directly and from the properties of the thing by means of the formal cause, can still be demonstrated in other ways, for instance, by means of its proper final cause, or even from its common principles, or indirectly either by what signifies it or by the method of reduction to absurdity. Thus St. Thomas in the books of the Contra Gentes makes use of these direct or indirect arguments so as to reach the same conclusion and places them together, not giving the reason why they are six or ten in number. But in the Summa theologica and the Quaestiones disputatate there is usually only one direct argument, which is of the formal kind and is deduced from the properties of a thing, introducing the proximately formal middle term, or if the holy Doctor gives two or three arguments he assigns the scientific reason why and how there are two or three methods of argumentation.
Therefore the middle term in the demonstration must be clearly presented, which in the syllogism of the first figure is the subject of the major and the predicate of the minor and we know that the modes of the other figures can be reduced to the modes of the first figure.
Therefore this middle term thus clearly stated presents itself as the keystone of the article, inserted in the syllogism as a precious jewel set in a ring. Thus we make use of logic, not indeed for its own sake, but that by it we may acquire a direct knowledge of the middle term or principle in which the truth of the conclusion must be considered, or at least of the main conclusion, if there are several conclusions in the article, as sometimes happens. Having accomplished this task to commit to memory what is of first importance in the article, it is enough to bear in mind the middle term. When the question is again posited, the major and minor terms are included in it; hence in replying to the question it suffices to enunciate the middle term in the demonstration, so that again we may have the demonstration of the conclusion. In illustration of this let us take the question: "Whether the human soul is incorruptible?" It suffices to reply to this: "Every simple and subsistent form . . . Therefore the human soul is incorruptible."
If the middle term in the demonstration of the article is thus carefully taken into consideration, this makes us see more clearly, without the aid of a syllogism, the solution of the objections which were presented in the beginning of the article. As a matter of fact, St. Thomas casts upon the solution of the objections the searchlight of the middle term in the demonstration, and by means of this light the distinction to be made is easily discovered and understood. After this, whatever doubts and corollaries there may be, these can be profitably presented. This method was often adopted by the Salmanticenses.
The stand taken by St. Thomas, if properly understood, is seen to be the just mean and summit between and above the two extremes: on the one hand, of empiric nominalism - which retains a certain objectivity of experience, though denying the necessity and universality of knowledge - and on the other, of the idealism of the conceptualists or subjectivists, which retains a certain necessity and universality of knowledge, although without any ontological validity, that is, without any true objectivity.
Thus St.Thomas' method of procedure in the construction of his articles is far more in accordance with the natural progress of the mind in its search for truth than is the method adopted by several Scholastics of a later date, who in the beginning multiply the preliminary remarks about those things that have already been explained by them and that do not need any further explanation. Often also they materially juxtapose these various preliminary remarks, not showing the essential relation between them, and then they propose the argument in the briefest manner, so that the middle term in the demonstration is not sufficiently clear, and sometimes several arguments in succession are proposed in which the direct formal argument deduced from the properties of a thing is not sufficiently distinguished from the others, or from those derived from the common principles, or from the indirect arguments. This later method is rather mechanical, whereas the method of St. Thomas is organic, according to the natural process of the mind in operation.
Lastly, the importance of the middle term in the demonstration is clearly perceived from the rules to be observed in scholastic disputatrons. The objector, in accordance with these rules, by clever argumentation, so as to overthrow the conclusion, must attack by three successive objections in scholastic form the middle term in the demonstration, which is, so to speak, the chief point of attack to be defended in the article, and, as it were, the citadel of the defender. But the defender of this citadel must train upon the objector the light of the middle term in the form of a brilliant distinction that is not accidentally but directly and truly to the point. Thus after a well-ordered scholastic demonstration, which is of reasonable difficulty, the truth of the article, having been sifted and freed of all its difficulties, becomes increasingly clear, and is certainly confirmed by this austere criticism which is, as it were, the acid that attacks all metals, gold alone excepted.
32. Post. Anal., Bk. I, chap. 2.
33. Summa theol., Ia, q.75, a.6.
34. Ibid., q.45, a.5.
35. Ibid., Ia IIae, q.2, a.8.
36. Ibid., IIa IIae, q. 1, a. 1, c.