Aquinas Day By Day


Aquinas’s topic: the parts of logic based on the certitude of our reasoning

Scripture: “Let your ‘yes’ mean ‘yes’ and your ‘no’ mean ‘no.’ Anything more is from the evil one.” Mt 5: 37

Aquinas’s text: Expositio libri Posteriorum Prologue

In a way, the acts of reason are similar to the acts of nature, so that art imitates nature, as far as it can. In the acts of nature is found an three-fold diversity. In some, nature acts necessarily, so that it cannot fail. In others, nature operates for the most part, though sometimes it can fail to perform its proper act. Consequently, in the latter there must be a two-fold act: one which happens for the most part, as when a perfect animal is generated from seed; and the other when nature fails to perform what is proper to it, as when from seed is generated something abnormal, owing to the corruption of some principle.

These three things are also found in the acts of reason. For there is one process of reasoning that leads to necessity, where it is not possible to fall short of the truth. Through this sort of reasoning process the certainty of science is acquired. Then there is another reasoning process where the truth is attained for the most part, but without achieving necessity. And there is a third reasoning process where reason falls short of the truth owing to not following some principle of reasoning.

The part of logic devoted to the first process is called the “judging part,” since there is judgment with the certitude of science. Now since certain judgment concerning effects can only be obtained by resolution to first principles, this part is called “analytical”, that is, “resolving.” Now the certitude in a judgment that is obtained by resolution, comes either from the form of the syllogism alone, and the book Prior Analytics, which is about the syllogism as such, is ordered to this end, or it comes from the matter along with the form, because it comes from propositions that are essential and necessary, and the book Posterior Analytics, which is about the demonstrative syllogism, is ordered to this end.

To the second process of reason is devoted another part of logic, which is called “inventive,” for finding things out is not always done with certitude. Consequently, about what has been found out we need judgment, in order to have certitude. Now just as in natural things which act for the most part there are certain levels attained, since the stronger the power of the nature the rarer does it fail in its effect, so likewise in the process of reasoning that is not completely certain, one finds certain levels wherein one attains more or less to perfect certitude. Through processes of this sort, even if one does not achieve science, sometimes one attains belief or opinion, owing to the probability of the propositions from which the argument proceeds. For reason completely takes one side of a contradiction, though with some fear of the other side. Now topics or dialectics is ordered to this end. For a dialectical syllogism proceeds from probable premisses, which Aristotle covers in his book of Topics. At other times, however, we do not completely achieve belief or opinion, but a kind of suspicion, since reason does not completely pick one side of a contradiction, though it is more inclined to this side rather than to that. Now rhetoric is devoted to this end. At other times only estimation inclines us to one part of a contradiction owing to some representation, in the way in which a human rejects some food when represented to him in an unpleasant way. Now poetics is ordered to this end, for the poet’s function is to lead us to something virtuous by means of some appropriate representation. Now all of these pertain to rational philosophy, for leading from one point to another is the function of reason.

Now the part of logic called “sophistical” is devoted to the third process of reason, which Aristotle treats in his book Sophistical Refutations.

Attendendum est autem quod actus rationis similes sunt, quantum ad aliquid, actibus naturae. Unde et ars imitatur naturam in quantum potest. In actibus autem naturae invenitur triplex diversitas. In quibusdam enim natura ex necessitate agit, ita quod non potest deficere. In quibusdam vero natura ut frequentius operatur, licet quandoque possit deficere a proprio actu. Unde in his necesse est esse duplicem actum; unum, qui sit ut in pluribus, sicut cum ex semine generatur animal perfectum; alium vero quando natura deficit ab eo quod est sibi conveniens, sicut cum ex semine generatur aliquod monstrum propter corruptionem alicuius principii.

Et haec etiam tria inveniuntur in actibus rationis. Est enim aliquis rationis processus necessitatem inducens, in quo non est possibile esse veritatis defectum; et per huiusmodi rationis processum scientiae certitudo acquiritur. Est autem alius rationis processus, in quo ut in pluribus verum concluditur, non tamen necessitatem habens. Tertius vero rationis processus est, in quo ratio a vero deficit propter alicuius principii defectum; quod in ratiocinando erat observandum.

Pars autem logicae, quae primo deservit processui, pars iudicativa dicitur, eo quod iudicium est cum certitudine scientiae. Et quia iudicium certum de effectibus haberi non potest nisi resolvendo in prima principia, ideo pars haec analytica vocatur, idest resolutoria. Certitudo autem iudicii, quae per resolutionem habetur, est, vel ex ipsa forma syllogismi tantum, et ad hoc ordinatur liber priorum analyticorum, qui est de syllogismo simpliciter; vel etiam cum hoc ex materia, quia sumuntur propositiones per se et necessariae, et ad hoc ordinatur liber posteriorum analyticorum, qui est de syllogismo demonstrativo.

Secundo autem rationis processui deservit alia pars logicae, quae dicitur inventiva. Nam inventio non semper est cum certitudine. Unde de his, quae inventa sunt, iudicium requiritur, ad hoc quod certitudo habeatur. Sicut autem in rebus naturalibus, in his quae ut in pluribus agunt, gradus quidam attenditur (quia quanto virtus naturae est fortior, tanto rarius deficit a suo effectu), ita et in processu rationis, qui non est cum omnimoda certitudine, gradus aliquis invenitur, secundum quod magis et minus ad perfectam certitudinem acceditur. Per huiusmodi enim processum, quandoque quidem, etsi non fiat scientia, fit tamen fides vel opinio propter probabilitatem propositionum, ex quibus proceditur: quia ratio totaliter declinat in unam partem contradictionis, licet cum formidine alterius, et ad hoc ordinatur topica sive dialectica. Nam syllogismus dialecticus ex probabilibus est, de quo agit Aristoteles in libro topicorum. Quandoque vero, non fit complete fides vel opinio, sed suspicio quaedam, quia non totaliter declinatur ad unam partem contradictionis, licet magis inclinetur in hanc quam in illam. Et ad hoc ordinatur rhetorica. Quandoque vero sola existimatio declinat in aliquam partem contradictionis propter aliquam repraesentationem, ad modum quo fit homini abominatio alicuius cibi, si repraesentetur ei sub similitudine alicuius abominabilis. Et ad hoc ordinatur poetica; nam poetae est inducere ad aliquod virtuosum per aliquam decentem repraesentationem. Omnia autem haec ad rationalem philosophiam pertinent: inducere enim ex uno in aliud rationis est.

Tertio autem processui rationis deservit pars logicae, quae dicitur sophistica, de qua agit Aristoteles in libro elenchorum.

[Introductions and translations © R.E. Houser]