|Aquinas Day by Day: Logic of Arguments: Three Features of Scientific Propositions|
Aquinas’ Topic: Logic of Arguments: Three Features of Scientific Propositions
Aquinas’ Text: Expositio libri Posteriorum , Bk. 1, lec. 9
Br. Thomas here introduces three fundamental features of the propositions that make up demonstrative syllogisms. Such propositions must be said “of all [de omni],” said “through themselves [per se],” which is often translated “essentially,” and they must be said “universally.”
After showing what a demonstrative syllogism is, the philosopher in this section begins to show from what and what kind of propositions it is.
He says therefore first (73a21), that since in the definition given above scientific knowledge cannot be otherwise, that which is scientifically known through demonstration will be necessary. Then he explains what it is to know something scientifically through demonstration, saying that demonstrative science is “what we have in having a demonstration,” that is, what we acquire through demonstration. So it follows that the conclusion of a demonstration is necessary. Although a necessary conclusion could be drawn syllogistically from contingent premisses, it is not possible to obtain scientific knowledge of the necessary from a contingent middle, as will be proved later. And since the conclusion of a demonstration is not only necessary but is known through demonstration, as has been said, it follows that a demonstrative syllogism proceeds from necessary premisses. Therefore, we must establish from what and from what sort of necessary premisses a demonstration proceeds.
First, he states his intention (73a25), saying that before determining specifically from what and what sort of premisses a demonstration comes, one must determine what is meant when we say, “of all,” and “through itself (per se),” and “universally.” For to understand these things, it is necessary to know from what premisses a demonstration comes; and to do this, we must look at demonstrations. For in the propositions of a demonstration it is necessary that something be predicated “universally,” which signifies being said “of all” and “through itself (per se),” and “first,” which signifies “universal.” Now these three things are related by adding something to the previous one. For whatever is predicated “through itself (per se )” is also predicated “of all,” but not the reverse. And also, whatever is predicated “universally” is predicated “through itself (per se),” but not the reverse. Therefore, this shows why they are arranged as they are.
Postquam philosophus ostendit quid sit syllogismus demonstrativus, in parte ista incipit ostendere ex quibus et qualibus sit. ..
Dicit ergo primo, quod quia dictum est supra, quod impossibile est aliter se habere in definitione eius quod est scire, necessarium erit id quod scitur secundum demonstrationem. Quid autem sit quod est secundum demonstrationem scire exponit, dicens quod demonstrativa scientia est quam habemus in habendo demonstrationem, idest quam ex demonstratione acquirimus. Et sic habetur quod demonstrationis conclusio sit necessaria. Quamvis autem necessarium possit syllogizari ex contingentibus, non tamen de necessario potest haberi scientia per medium contingens, ut infra probabitur. Et quia conclusio demonstrationis non solum est necessaria, sed etiam per demonstrationem scita, ut dictum est, sequitur quod demonstrativus syllogismus sit ex necessariis. Et ideo accipiendum est ex quibus necessariis et qualibus sint demonstrationis.
Primo, dicit de quo est intentio, dicens quod antequam determinetur in speciali ex quibus et qualibus sit demonstratio, primo determinandum est quid intelligatur cum dicimus de omni, et per se, et universale. Cognoscere enim ista est necessarium ad sciendum ex quibus sit demonstratio. Hoc namque oportet observari in demonstrationibus. Oportet enim in propositionibus demonstrationis aliquid universaliter praedicari, quod significat dici de omni, et per se, et etiam primo, quod significat universale. Haec autem tria se habent ex additione ad invicem. Nam omne quod per se praedicatur, etiam universaliter praedicatur; sed non e converso. Similiter omne quod primo praedicatur, praedicatur per se, sed non convertitur. Unde etiam apparet ratio ordinis istorum.
[Introductions and translations © R.E. Houser]