The Ways to Wisdom
The Ways to Wisdom
Aquinas Day By Day


Aquinas’s topic:  logic of arguments:  reasoning is deductive or inductive


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

The third act of the mind is the act of reasoning, the process of learning something new by putting together information we already possess by means of the first two mental acts.  Aristotle had begun his treatise on “scientific knowledge,” the Posterior Analytics , by noting that the activity of reasoning always moves from something we already know:  “All teaching and all learning of an intellectual sort proceeds from pre-existent knowledge.”  Aristotle then went on to illustrate this claim with examples from the realms of “scientific knowledge,” “dialectics” or logic, and “rhetoric.”  In his commentary, Br. Thomas points out that Aristotle’s argument is inductive, and he emphasizes the difference between deductive or syllogistic argument and inductive argument.  These are the two kinds of reasoning.  Both are based on pre-existing knowledge, but not the same kind of pre-existing knowledge.

Then (71a3) [Aristotle] manifests his claim inductively.  First, he does so in demonstrative arguments by which scientific knowledge is acquired. Of these, the best are the mathematical sciences, because their mode of demonstration is absolutely certain.  After them come all the other disciplines, because in all of them there is some mode of demonstration; otherwise they would not be sciences.

Second (71a4), he manifests the same claim in disputations or dialectical discourse, which makes use of syllogism and induction, both of which proceed from something already known. For in a syllogism the knowledge of some universal conclusion is obtained from other universals already known; while in induction a universal is drawn as a conclusion from singulars that are manifest to sense.

Third (71a8), he manifests the same claim in rhetorical discourse, where persuasion is produced through enthymeme or example, not through syllogism or complete induction, because of the uncertainty of the matters at issue, namely, the individual acts of humans, where universal propositions cannot be truly drawn. Therefore, in place of a syllogism where there must be some universal, one uses an enthymeme. Likewise, in place of induction where one concludes to a universal, an example is used, where one goes from a singular, not to a universal, but to a singular. Consequently, it is clear that just as an enthymeme is a kind of abridged syllogism, so too an example is a kind of incomplete induction. Therefore, if in syllogism and induction one proceeds from something previously known, the same thing must be understood of the enthymeme and example.

Secundo, cum dicit: mathematicae enim etc., manifestat propositionem praemissam per inductionem. Et primo, in demonstrativis in quibus acquiritur scientia. In his autem principaliores sunt mathematicae scientiae, propter certissimum modum demonstrationis. Consequenter autem sunt et omnes aliae artes, quia in omnibus est aliquis modus demonstrationis, alias non essent scientiae.

Secundo, cum dicit: similiter autem etc., manifestat idem in orationibus disputativis sive dialecticis, quae utuntur syllogismo et inductione: in quorum utroque proceditur ex aliquo praecognito. Nam in syllogismo accipitur cognitio alicuius universalis conclusi ab aliis universalibus notis. In inductione autem concluditur universale ex singularibus, quae sunt manifesta quantum ad sensum.

Tertio, cum dicit: similiter autem rhetoricae etc., manifestat idem in rhetoricis, in quibus persuasio fit per enthymema aut per exemplum; non autem per syllogismum vel inductionem completam, propter incertitudinem materiae circa quam versatur, scilicet circa actus singulares hominum, in quibus universales propositiones non possunt assumi vere. Et ideo utitur loco syllogismi, in quo necesse est esse aliquam universalem, aliquo enthymemate; et similiter loco inductionis, in qua concluditur universale, aliquo exemplo, in quo proceditur a singulari, non ad universale, sed ad singulare. Unde patet quod, sicut enthymema est quidam syllogismus detruncatus, ita exemplum est quaedam inductio imperfecta. Si ergo in syllogismo et inductione proceditur ex aliquo praecognito, oportet idem intelligi in enthymemate et exemplo.

[Introductions and translations © R.E. Houser]

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