1 Objection 1: It would seem that the intellect can be false; for the Philosopher says Metaph. vi, Did. v, 4) that truth and falsehood are in the mind. But the mind and intellect are the same, as is shown above Question 79, Article 1). Therefore falsehood may be in the mind.
1 Ad sextum sic proceditur. Videtur quod intellectus possit esse falsus. Dicit enim philosophus, in VI Metaphys., quod verum et falsum sunt in mente. Mens autem et intellectus idem sunt, ut supra dictum est. Ergo falsitas est in intellectu.
2 Objection 2: Further, opinion and reasoning belong to the intellect. But falsehood exists in both. Therefore falsehood can be in the intellect.
2 Praeterea, opinio et ratiocinatio ad intellectum pertinent. Sed in utraque istarum invenitur falsitas. Ergo potest esse falsitas in intellectu.
3 Objection 3: Further, sin is in the intellectual faculty. But sin involves falsehood: for those err that work evil Prov. 14, 22). Therefore falsehood can be in the intellect.
3 Praeterea, peccatum in parte intellectiva est. Sed peccatum cum falsitate est, errant enim qui operantur malum, ut dicitur Prov. XIV. Ergo falsitas potest esse in intellectu.
4 On the contrary, Augustine says Questions. 83, qu. 32), that everyone who is deceived, does not rightly understand that wherein he is deceived. And the Philosopher says De Anima iii, 10), that the intellect is always true.
4 Sed contra est quod dicit Augustinus, in libro octoginta trium quaest., quod omnis qui fallitur, id in quo fallitur non intelligit. Et philosophus dicit, in libro de anima, quod intellectus semper est rectus.
5 I answer that, The Philosopher De Anima iii, 6) compares intellect with sense on this point. For sense is not deceived in its proper object, as sight in regard to color; has accidentally through some hindrance occurring to the sensile organ, for example, the taste of a fever-stricken person judges a sweet thing to be bitter, through his tongue being vitiated by ill humors. Sense, however, may be deceived as regards common sensible objects, as size or figure; when, for example, it judges the sun to be only a foot in diameter, whereas in reality it exceeds the earth in size.
Much more is sense deceived concerning accidental sensible objects, as when it judges that vinegar is honey by reason of the color being the same. The reason of this is evident; for every faculty, as such, is per se directed to its proper object; and things of this kind are always the same. Hence, as long as the faculty exists, its judgment concerning its own proper object does not fail. Now the proper object of the intellect is the quiddity of a material thing; and hence, properly speaking, the intellect is not at fault concerning this quiddity; whereas it may go astray as regards the surroundings of the thing in its essence or quiddity, in referring one thing to another, as regards composition or division, or also in the process of reasoning.
Therefore, also in regard to those propositions, which are understood, the intellect cannot err, as in the case of first principles from which arises infallible truth in the certitude of scientific conclusions. The intellect, however, may be accidentally deceived in the quiddity of composite things, not by the defect of its organ, for the intellect is a faculty that is independent of an organ; but on the part of the composition affecting the definition, when, for instance, the definition of a thing is false in relation to something else, as the definition of a circle applied to a triangle; or when a definition is false in itself as involving the composition of things incompatible; as, for instance, to describe anything as a rational winged animal.
Hence as regards simple objects not subject to composite definitions we cannot be deceived unless, indeed, we understand nothing whatever about them, as is said Metaph. ix, Did. viii, 10.
5 Respondeo dicendum quod philosophus, in III de anima, comparat, quantum ad hoc, intellectum sensui. Sensus enim circa proprium obiectum non decipitur, sicut visus circa colorem; nisi forte per accidens, ex impedimento circa organum contingente, sicut cum gustus febrientium dulcia iudicat amara, propter hoc quod lingua malis humoribus est repleta. Circa sensibilia vero communia decipitur sensus, sicut in diiudicando de magnitudine vel figura; ut cum iudicat solem esse pedalem, qui tamen est maior terra. Et multo magis decipitur circa sensibilia per accidens; ut cum iudicat fel esse mel, propter coloris similitudinem.
Et huius ratio est in evidenti. Quia ad proprium obiectum unaquaeque potentia per se ordinatur, secundum quod ipsa. Quae autem sunt huiusmodi, semper eodem modo se habent. Unde manente potentia, non deficit eius iudicium circa proprium obiectum. Obiectum autem proprium intellectus est quidditas rei. Unde circa quidditatem rei, per se loquendo, intellectus non fallitur. Sed circa ea quae circumstant rei essentiam vel quidditatem, intellectus potest falli, dum unum ordinat ad aliud, vel componendo vel dividendo vel etiam ratiocinando. Et propter hoc etiam circa illas propositiones errare non potest, quae statim cognoscuntur cognita terminorum quidditate, sicut accidit circa prima principia, ex quibus etiam accidit infallibilitas veritatis, secundum certitudinem scientiae, circa conclusiones.
Per accidens tamen contingit intellectum decipi circa quod quid est in rebus compositis; non ex parte organi, quia intellectus non est virtus utens organo; sed ex parte compositionis intervenientis circa definitionem, dum vel definitio unius rei est falsa de alia, sicut definitio circuli de triangulo, vel dum aliqua definitio in seipsa est falsa, implicans compositionem impossibilium, ut si accipiatur hoc ut definitio alicuius rei, animal rationale alatum. Unde in rebus simplicibus, in quarum definitionibus compositio intervenire non potest, non possumus decipi; sed deficimus in totaliter non attingendo, sicut dicitur in IX Metaphys.
6 Reply to Objection 1: The Philosopher says that falsehood is in the intellect in regard to composition and division. The same answer applies to the Second Objection concerning opinion and reasoning, and to the Third Objection, concerning the error of the sinner, who errs in the practical judgment of the appetible object. But in the absolute consideration of the quiddity of a thing, and of those things which are known thereby, the intellect is never deceived. In this sense are to be understood the authorities quoted in proof of the opposite conclusion.
6 Ad primum ergo dicendum quod falsitatem dicit esse philosophus in mente secundum compositionem et divisionem. Et similiter dicendum est ad secundum, de opinione et ratiocinatione. Et ad tertium, de errore peccantium, qui consistit in applicatione ad appetibile. Sed in absoluta consideratione quidditatis rei, et eorum quae per eam cognoscuntur, intellectus nunquam decipitur. Et sic loquuntur auctoritates in contrarium inductae.