Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that the soul is a body. For the soul is the moving principle of the body. Nor does it move unless moved. First, because seemingly nothing can move unless it is itself moved, since nothing gives what it has not; for instance, what is not hot does not give heat. Secondly, because if there be anything that moves and is not moved, it must be the cause of eternal, unchanging movement, as we find proved Phys. viii, 6; and this does not appear to be the case in the movement of an animal, which is caused by the soul. Therefore the soul is a mover moved. But every mover moved is a body. Therefore the soul is a body.

Objection 2: Further, all knowledge is caused by means of a likeness. But there can be no likeness of a body to an incorporeal thing. If, therefore, the soul were not a body, it could not have knowledge of corporeal things.

Objection 3: Further, between the mover and the moved there must be contact. But contact is only between bodies. Since, therefore, the soul moves the body, it seems that the soul must be a body.

On the contrary, Augustine says (De Trin. vi, 6) that the soul "is simple in comparison with the body, inasmuch as it does not occupy space by its bulk."

I answer that, To seek the nature of the soul, we must premise that the soul is defined as the first principle of life of those things which live: for we call living things "animate," , and those things which have no life, "inanimate." Now life is shown principally by two actions, knowledge and movement. The philosophers of old, not being able to rise above their imagination, supposed that the principle of these actions was something corporeal: for they asserted that only bodies were real things; and that what is not corporeal is nothing: hence they maintained that the soul is something corporeal. This opinion can be proved to be false in many ways; but we shall make use of only one proof, based on universal and certain principles, which shows clearly that the soul is not a body.

It is manifest that not every principle of vital action is a soul, for then the eye would be a soul, as it is a principle of vision; and the same might be applied to the other instruments of the soul: but it is the "first" principle of life, which we call the soul. Now, though a body may be a principle of life, or to be a living thing, as the heart is a principle of life in an animal, yet nothing corporeal can be the first principle of life. For it is clear that to be a principle of life, or to be a living thing, does not belong to a body as such; since, if that were the case, every body would be a living thing, or a principle of life. Therefore a body is competent to be a living thing or even a principle of life, as "such" a body. Now that it is actually such a body, it owes to some principle which is called its act. Therefore the soul, which is the first principle of life, is not a body, but the act of a body; thus heat, which is the principle of calefaction, is not a body, but an act of a body.

Reply to Objection 1: As everything which is in motion must be moved by something else, a process which cannot be prolonged indefinitely, we must allow that not every mover is moved. For, since to be moved is to pass from potentiality to actuality, the mover gives what it has to the thing moved, inasmuch as it causes it to be in act. But, as is shown in Phys. viii, 6, there is a mover which is altogether immovable, and not moved either essentially, or accidentally; and such a mover can cause an invariable movement. There is, however, another kind of mover, which, though not moved essentially, is moved accidentally; and for this reason it does not cause an invariable movement; such a mover, is the soul. There is, again, another mover, which is moved essentially---namely, the body. And because the philosophers of old believed that nothing existed but bodies, they maintained that every mover is moved; and that the soul is moved directly, and is a body.

Reply to Objection 2: The likeness of a thing known is not of necessity actually in the nature of the knower; but given a thing which knows potentially, and afterwards knows actually, the likeness of the thing known must be in the nature of the knower, not actually, but only potentially; thus color is not actually in the pupil of the eye, but only potentially. Hence it is necessary, not that the likeness of corporeal things should be actually in the nature of the soul, but that there be a potentiality in the soul for such a likeness. But the ancient philosophers omitted to distinguish between actuality and potentiality; and so they held that the soul must be a body in order to have knowledge of a body; and that it must be composed of the principles of which all bodies are formed in order to know all bodies.

Reply to Objection 3: There are two kinds of contact; of "quantity," and of "power." By the former a body can be touched only by a body; by the latter a body can be touched by an incorporeal thing, which moves that body.

Articulus 1

Argumentum 1

Ad primum sic proceditur. Videtur quod anima sit corpus.

Anima enim est motor corporis. Non autem est movens non motum.

Tum quia videtur quod nihil possit movere nisi moveatur, quia nihil dat alteri quod non habet, sicut quod non est calidum non calefacit. Tum quia, si aliquid est movens non motum, causat motum sempiternum et eodem modo se habentem, ut probatur in VIII physic., quod non apparet in motu animalis, qui est ab anima. Ergo anima est movens motum. Sed omne movens motum est corpus. Ergo anima est corpus.

Argumentum 2

Praeterea, omnis cognitio fit per aliquam similitudinem.

Non potest autem esse similitudo corporis ad rem incorpoream.

Si igitur anima non esset corpus, non posset cognoscere res corporeas.

Argumentum 3

Praeterea, moventis ad motum oportet esse aliquem contactum.

Contactus autem non est nisi corporum. Cum igitur anima moveat corpus, videtur quod anima sit corpus.

Sed Contra

Sed contra est quod Augustinus dicit, VI de Trin., quod anima simplex dicitur respectu corporis, quia mole non diffunditur per spatium loci.

Corpus

Respondeo dicendum quod ad inquirendum de natura animae, oportet praesupponere quod anima dicitur esse primum principium vitae in his quae apud nos vivunt animata enim viventia dicimus, res vero inanimatas vita carentes. Vita autem maxime manifestatur duplici opere, scilicet cognitionis et motus.

Horum autem principium antiqui philosophi, imaginationem transcendere non valentes, aliquod corpus ponebant; sola corpora res esse dicentes, et quod non est corpus, nihil esse. Et secundum hoc, animam aliquod corpus esse dicebant.

Huius autem opinionis falsitas licet multipliciter ostendi possit tamen uno utemur, quo et communius et certius patet animam corpus non esse. Manifestum est enim quod non quodcumque vitalis operationis principium est anima, sic enim oculus esset anima, cum sit quoddam principium visionis; et idem esset dicendum de aliis animae instrumentis. Sed primum principium vitae dicimus esse animam. Quamvis autem aliquod corpus possit esse quoddam principium vitae, sicut cor est principium vitae in animali; tamen non potest esse primum principium vitae aliquod corpus.

Manifestum est enim quod esse principium vitae, vel vivens, non convenit corpori ex hoc quod est corpus, alioquin omne corpus esset vivens, aut principium vitae.

Convenit igitur alicui corpori quod sit vivens, vel etiam principium vitae, per hoc quod est tale corpus.

Quod autem est actu tale, habet hoc ab aliquo principio quod dicitur actus eius. Anima igitur, quae est primum principium vitae, non est corpus, sed corporis actus, sicut calor, qui est principium calefactionis, non est corpus, sed quidam corporis actus.

Ad 1

Ad primum ergo dicendum quod, cum omne quod movetur ab alio moveatur, quod non potest in infinitum procedere, necesse est dicere quod non omne movens movetur. Cum enim moveri sit exire de potentia in actum, movens dat id quod habet mobili, inquantum facit ipsum esse in actu.

Sed sicut ostenditur in VIII physic., est quoddam movens penitus immobile, quod nec per se nec per accidens movetur, et tale movens potest movere motum semper uniformem. Est autem aliud movens, quod non movetur per se, sed movetur per accidens, et propter hoc non movet motum semper uniformem. Et tale movens est anima. Est autem aliud movens, quod per se movetur, scilicet corpus.

Et quia antiqui naturales nihil esse credebant nisi corpora, posuerunt quod omne movens movetur, et quod anima per se movetur, et est corpus.

Ad 2

Ad secundum dicendum quod non est necessarium quod similitudo rei cognitae sit actu in natura cognoscentis, sed si aliquid sit quod prius est cognoscens in potentia et postea in actu, oportet quod similitudo cogniti non sit actu in natura cognoscentis, sed in potentia tantum; sicut color non est actu in pupilla, sed in potentia tantum. Unde non oportet quod in natura animae sit similitudo rerum corporearum in actu; sed quod sit in potentia ad huiusmodi similitudines. Sed quia antiqui naturales nesciebant distinguere inter actum et potentiam, ponebant animam esse corpus, ad hoc quod cognosceret corpus; et ad hoc quod cognosceret omnia corpora, quod esset composita ex principiis omnium corporum.

Ad 3

Ad tertium dicendum quod est duplex contactus, quantitatis et virtutis. Primo modo, corpus non tangitur nisi a corpore. Secundo modo, corpus potest tangi a re incorporea quae movet corpus.