Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that the soul does not know bodies through the intellect. For Augustine says (Soliloq. ii, 4) that "bodies cannot be understood by the intellect; nor indeed anything corporeal unless it can be perceived by the senses." He says also (Gen. ad lit. xii, 24) that intellectual vision is of those things that are in the soul by their essence. But such are not bodies. Therefore the soul cannot know bodies through the intellect.

Objection 2: Further, as sense is to the intelligible, so is the intellect to the sensible. But the soul can by no means, through the senses, understand spiritual things, which are intelligible. Therefore by no means can it, through the intellect, know bodies, which are sensible.

Objection 3: Further, the intellect is concerned with things that are necessary and unchangeable. But all bodies are mobile and changeable. Therefore the soul cannot know bodies through the intellect.

On the contrary, Science is in the intellect. If, therefore, the intellect does not know bodies, it follows that there is no science of bodies; and thus perishes natural science, which treats of mobile bodies.

I answer that, It should be said in order to elucidate this question, that the early philosophers, who inquired into the natures of things, thought there was nothing in the world save bodies. And because they observed that all bodies are mobile, and considered them to be ever in a state of flux, they were of opinion that we can have no certain knowledge of the true nature of things. For what is in a continual state of flux, cannot be grasped with any degree of certitude, for it passes away ere the mind can form a judgment thereon: according to the saying of Heraclitus, that "it is not possible twice to touch a drop of water in a passing torrent," as the Philosopher relates (Metaph. iv, Did. iii, 5).

After these came Plato, who, wishing to save the certitude of our knowledge of truth through the intellect, maintained that, besides these things corporeal, there is another genus of beings, separate from matter and movement, which beings he called "species" or "ideas," by participation of which each one of these singular and sensible things is said to be either a man, or a horse, or the like. Wherefore he said that sciences and definitions, and whatever appertains to the act of the intellect, are not referred to these sensible bodies, but to those beings immaterial and separate: so that according to this the soul does not understand these corporeal things, but the separate species thereof.

Now this may be shown to be false for two reasons. First, because, since those species are immaterial and immovable, knowledge of movement and matter would be excluded from science (which knowledge is proper to natural science), and likewise all demonstration through moving and material causes. Secondly, because it seems ridiculous, when we seek for knowledge of things which are to us manifest, to introduce other beings, which cannot be the substance of those others, since they differ from them essentially: so that granted that we have a knowledge of those separate substances, we cannot for that reason claim to form a judgment concerning these sensible things.

Now it seems that Plato strayed from the truth because, having observed that all knowledge takes place through some kind of similitude, he thought that the form of the thing known must of necessity be in the knower in the same manner as in the thing known. Then he observed that the form of the thing understood is in the intellect under conditions of universality, immateriality, and immobility: which is apparent from the very operation of the intellect, whose act of understanding has a universal extension, and is subject to a certain amount of necessity: for the mode of action corresponds to the mode of the agent's form. Wherefore he concluded that the things which we understand must have in themselves an existence under the same conditions of immateriality and immobility.

But there is no necessity for this. For even in sensible things it is to be observed that the form is otherwise in one sensible than in another: for instance, whiteness may be of great intensity in one, and of a less intensity in another: in one we find whiteness with sweetness, in another without sweetness. In the same way the sensible form is conditioned differently in the thing which is external to the soul, and in the senses which receive the forms of sensible things without receiving matter, such as the color of gold without receiving gold. So also the intellect, according to its own mode, receives under conditions of immateriality and immobility, the species of material and mobile bodies: for the received is in the receiver according to the mode of the receiver. We must conclude, therefore, that through the intellect the soul knows bodies by a knowledge which is immaterial, universal, and necessary.

Reply to Objection 1: These words of Augustine are to be understood as referring to the medium of intellectual knowledge, and not to its object. For the intellect knows bodies by understanding them, not indeed through bodies, nor through material and corporeal species; but through immaterial and intelligible species, which can be in the soul by their own essence.

Reply to Objection 2: As Augustine says (De Civ. Dei xxii, 29), it is not correct to say that as the sense knows only bodies so the intellect knows only spiritual things; for it follows that God and the angels would not know corporeal things. The reason of this diversity is that the lower power does not extend to those things that belong to the higher power; whereas the higher power operates in a more excellent manner those things which belong to the lower power.

Reply to Objection 3: Every movement presupposes something immovable: for when a change of quality occurs, the substance remains unmoved; and when there is a change of substantial form, matter remains unmoved. Moreover the various conditions of mutable things are themselves immovable; for instance, though Socrates be not always sitting, yet it is an immovable truth that whenever he does sit he remains in one place. For this reason there is nothing to hinder our having an immovable science of movable things.

Articulus 1

Argumentum 1

Ad primum sic proceditur. Videtur quod anima non cognoscat corpora per intellectum.

Dicit enim Augustinus, in II soliloq. Quod corpora intellectu comprehendi non possunt; nec aliquod corporeum nisi sensibus videri potest.

Dicit etiam, XII super Gen. Ad litt., quod visio intellectualis est eorum quae sunt per essentiam suam in anima.

Huiusmodi autem non sunt corpora.

Ergo anima per intellectum corpora cognoscere non potest.

Argumentum 2

Praeterea, sicut se habet sensus ad intelligibilia, ita se habet intellectus ad sensibilia. Sed anima per sensum nullo modo potest cognoscere spiritualia, quae sunt intelligibilia. Ergo nullo modo per intellectum potest cognoscere corpora, quae sunt sensibilia.

Argumentum 3

Praeterea, intellectus est necessariorum et semper eodem modo se habentium. Sed corpora omnia sunt mobilia, et non eodem modo se habentia. Anima ergo per intellectum corpora cognoscere non potest.

Sed Contra

Sed contra est quod scientia est in intellectu. Si ergo intellectus non cognoscit corpora, sequitur quod nulla scientia sit de corporibus.

Et sic peribit scientia naturalis, quae est de corpore mobili.

Corpus

Respondeo dicendum, ad evidentiam huius quaestionis, quod primi philosophi qui de naturis rerum inquisiverunt, putaverunt nihil esse in mundo praeter corpus. Et quia videbant omnia corpora mobilia esse, et putabant ea in continuo fluxu esse, aestimaverunt quod nulla certitudo de rerum veritate haberi posset a nobis.

Quod enim est in continuo fluxu, per certitudinem apprehendi non potest, quia prius labitur quam mente diiudicetur, sicut Heraclitus dixit quod non est possibile aquam fluvii currentis bis tangere, ut recitat philosophus in IV metaphys.. His autem superveniens Plato, ut posset salvare certam cognitionem veritatis a nobis per intellectum haberi, posuit praeter ista corporalia aliud genus entium a materia et motu separatum, quod nominabat species sive ideas, per quarum participationem unumquodque istorum singularium et sensibilium dicitur vel homo vel equus vel aliquid huiusmodi. Sic ergo dicebat scientias et definitiones et quidquid ad actum intellectus pertinet, non referri ad ista corpora sensibilia, sed ad illa immaterialia et separata; ut sic anima non intelligat ista corporalia, sed intelligat horum corporalium species separatas.

Sed hoc dupliciter apparet falsum. Primo quidem quia, cum illae species sint immateriales et immobiles, excluderetur a scientiis cognitio motus et materiae (quod est proprium scientiae naturalis) et demonstratio per causas moventes et materiales. Secundo autem, quia derisibile videtur ut, dum rerum quae nobis manifestae sunt notitiam quaerimus, alia entia in medium afferamus, quae non possunt esse earum substantiae, cum ab eis differant secundum esse, et sic, illis substantiis separatis cognitis, non propter hoc de istis sensibilibus iudicare possemus.

Videtur autem in hoc Plato deviasse a veritate, quia, cum aestimaret omnem cognitionem per modum alicuius similitudinis esse, credidit quod forma cogniti ex necessitate sit in cognoscente eo modo quo est in cognito. Consideravit autem quod forma rei intellectae est in intellectu universaliter et immaterialiter et immobiliter, quod ex ipsa operatione intellectus apparet, qui intelligit universaliter et per modum necessitatis cuiusdam; modus enim actionis est secundum modum formae agentis. Et ideo existimavit quod oporteret res intellectas hoc modo in seipsis subsistere, scilicet immaterialiter et immobiliter.

Hoc autem necessarium non est. Quia etiam in ipsis sensibilibus videmus quod forma alio modo est in uno sensibilium quam in altero, puta cum in uno est albedo intensior, in alio remissior, et in uno est albedo cum dulcedine, in alio sine dulcedine.

Et per hunc etiam modum forma sensibilis alio modo est in re quae est extra animam, et alio modo in sensu, qui suscipit formas sensibilium absque materia, sicut colorem auri sine auro.

Et similiter intellectus species, corporum, quae sunt materiales et mobiles, recipit immaterialiter et immobiliter, secundum modum suum, nam receptum est in recipiente per modum recipientis.

Dicendum est ergo quod anima per intellectum cognoscit corpora cognitione immateriali, universali et necessaria.

Ad 1

Ad primum ergo dicendum quod verbum Augustini est intelligendum quantum ad ea quibus intellectus cognoscit, non autem quantum ad ea quae cognoscit. Cognoscit enim corpora intelligendo, sed non per corpora, neque per similitudines materiales et corporeas; sed per species immateriales et intelligibiles, quae per sui essentiam in anima esse possunt.

Ad 2

Ad secundum dicendum quod, sicut Augustinus dicit XXII de Civit. Dei, non est dicendum quod, sicut sensus cognoscit sola corporalia, ita intellectus cognoscit sola spiritualia, quia sequeretur quod deus et Angeli corporalia non cognoscerent. Huius autem diversitatis ratio est, quia inferior virtus non se extendit ad ea quae sunt superioris virtutis; sed virtus superior ea quae sunt inferioris virtutis, excellentiori modo operatur.

Ad 3

Ad tertium dicendum quod omnis motus supponit aliquid immobile, cum enim transmutatio fit secundum qualitatem, remanet substantia immobilis; et cum transmutatur forma substantialis, remanet materia immobilis. Rerum etiam mutabilium sunt immobiles habitudines, sicut socrates etsi non semper sedeat, tamen immobiliter est verum quod, quandocumque sedet, in uno loco manet. Et propter hoc nihil prohibet de rebus mobilibus immobilem scientiam habere.