The Ways to Wisdom
Aquinas Day By Day


Aquinas’s topic:  logic of arguments:  demonstration of singulars


Aquinas’s text:  Expositio libri Posteriorum , Bk. 1, lec. 16

Here is Br. Thomas’s explanation of the second fpart of Aristotle’s argument in Posterior Analytics 1.7, showing how it is possible to have demonstrative knowledge of the changing things in the physical world.  Br. Thomas begins by showing he is aware that the text contains Aristotle’s answer to Plato.

Here one must understand that because demonstration is not about destructible but eternal things, and definition is, too, Plato was led to posit the “ideas.” Since sensible things are destructible, it seemed that there could be no demonstration or definition of them. Therefore, it seemed necessary to posit certain indestructible substances, about which demonstrations and definitions are given. These eternal substances he called “species” or “ideas.”

But against this opinion Aristotle was opposed, when he said above that demonstration is not of destructible things, except incidentally.  For even if these sensible things are destructible as individuals, in the universal they have a kind of eternity.  Therefore, since demonstration is given about these sensible things universally but not individually, it follows that demonstration is not about destructible things, except incidentally, but it is about eternal things essentially.

Then (75b32) [Aristotle] shows how there can be demonstration of things that occur frequently, saying that “of things that occur often, there are science and demonstrations, as of the eclipse of the moon,” which does not happen always. For the moon is not always eclipsed, but only sometimes. Now things that occur frequently, “so far as they are such,” that is, so far as demonstrations are given about them, are always; but so far as they are not always, they are particular.  Now about particulars there can be no demonstration, as was shown above, but only about universals. Consequently, it is clear that these sorts of things, in so far as there is demonstration about them, are always. And just as in the eclipse of the moon, so also for other similar things.

However, there are certain differences to consider in these cases. For some things are not always in time, but they are always in relation to their cause, because it never fails that when such a cause is given the effect follows, as in the eclipse of the moon. For it never fails that there is always an eclipse of the moon when the earth is diametrically interposed between sun and moon. But for other things it happens that they are not always, even in relation to their cause, because the causes can be impeded. For from a human seed a human with two hands is not always generated, but sometimes a failure occurs, owing to a defect either in the efficient cause or material cause. However, in both such cases demonstrations must be set up so from universal propositions is inferred a universal conclusion, by ruling out those things where there can be a defect, either on the side of time alone, or also on the side of some cause.

Sciendum est autem quod quia demonstratio non est corruptibilium, sed sempiternorum, neque definitio, Plato coactus fuit ponere ideas. Cum enim ista sensibilia sint corruptibilia, videbatur quod eorum non posset esse neque demonstratio, neque definitio. Et ideo videbatur quod oporteret ponere quasdam substantias incorruptibiles, de quibus et demonstrationes et definitiones darentur. Et has substantias sempiternas vocabat species vel ideas.

Sed huic opinioni occurrit Aristoteles superius dicens quod demonstratio non est corruptibilium nisi per accidens. Etsi enim ista sensibilia corruptibilia sint in particulari, in universali tamen quamdam sempiternitatem habent. Cum ergo demonstratio detur de istis sensibilibus in universali, non autem in particulari, sequitur quod demonstratio non sit corruptibilium, nisi per accidens; sempiternorum autem est per se.

Deinde cum dicit: eorum autem quae etc., ostendit quomodo eorum, quae sunt ut frequenter, possit esse demonstratio, dicens: quod eorum quae saepe fiunt, sunt etiam demonstrationes et scientiae: sicut de defectu lunae, qui tamen non semper est. Non enim luna semper deficit, sed aliquando. Haec autem quae sunt frequenter, secundum quod huiusmodi sunt, idest secundum quod de eis demonstrationes dantur, sunt semper: sed secundum quod non sunt semper, sunt particularia. De particularibus autem non potest esse demonstratio, ut ostensum est, sed solum de universalibus. Unde patet quod huiusmodi, secundum quod de eis est demonstratio, sunt semper. Et sicut est de defectu lunae, ita est de omnibus aliis similibus. Consideranda tamen est differentia inter ea. Quaedam enim non sunt semper secundum tempus, sunt autem semper per comparationem ad causam: quia nunquam deficit, quin posita tali causa, sequatur effectus; sicut est de defectu lunae. Nunquam enim deficit, quin semper sit lunae eclypsis, quandocunque terra diametraliter interponitur inter solem et lunam. In quibusdam vero contingit quod non semper sunt, etiam per comparationem ad causam: quia videlicet causae impediri possunt. Non enim semper ex semine hominis generatur homo habens duas manus; sed quandoque fit defectus vel propter impedimentum causae agentis vel materiae. In utrisque autem sic ordinandae sunt demonstrationes, ut ex universalibus propositionibus inferatur universalis conclusio, removendo illa, in quibus potest esse defectus vel ex parte temporis tantum vel etiam ex parte causae.

[Introductions and translations © R.E. Houser]

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