(A note on Thomas Aquinas’ “Summa Theologica”)
I am imagining Thomas Aquinas sitting at his desk, writing a chapter of his “Summa Theologica”…
Aquinas derives his theory of human nature from Aristotle’s “De Anima”. In the antique view, as can be found in Aristotle, what was called “psyche” (ψυχή) in Greek or “anima”in Latin, often translated as “soul” is the life-giving principle that distinguishes a dead body from a living one. Using the translation “soul” here is a bit misleading since the meaning of this concept changed in early modern times. For example, Descartes distinguished a material “res extensa” (extended thing) from what he called “res cogitans” (thinking thing). A living animal in Descartes sense is res extensa. Here, the “soul” no longer is the life-giving principle but it is restricted to those aspects of the human that we would connect with what we call the “mind”. In antiquity, on the other hand, the concept of “anima” had a different meaning. In Aristotle’s theory of “hylomorphism”, in which being is seen as a “compound” of matter and form, the “soul” can be viewed as the form of a living thing. Aristotle distinguished between three forms of psyche (vegetal, animal and rational, with the rational one being restricted to human beings). From Descartes on, the concept of psyche was then narrowed down to this “rational” soul.
It is this rational soul that interests Aquinas. One could argue that when a living thing dies and the psyche is its form, this form is destroyed. Aristotle himself seems a bit murky about this question. This, of course, poses a problem for a Christian theologian and philosopher like Thomas Aquinas who gets his conceptual toolkit manly from Aristotle.
In his Summa Theologica, Thomas Aquinas argues that the human intellect is something incorporeal. We see him here mid-way in the development between the antique concept of anima as the life-giving principle and the more recent notion of it as a res cogitans. He is, obviously, not interested in the vegetal or animal soul here since he speaks of the “principle of intellectual operation which we call the soul” (principium intellectualis operationis, quod dicimus animam hominis) In Summa Theologiae (Ia.75.2 ad1) we read:
I answer that, It must necessarily be allowed that the principle of intellectual operation which we call the soul, is a principle both incorporeal and subsistent. For it is clear that by means of the intellect man can have knowledge of all corporeal things. Now whatever knows certain things cannot have any of them in its own nature; because that which is in it naturally would impede the knowledge of anything else. Thus we observe that a sick man’s tongue being vitiated by a feverish and bitter humor, is insensible to anything sweet, and everything seems bitter to it. Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies. Now every body has its own determinate nature. Therefore it is impossible for the intellectual principle to be a body. It is likewise impossible for it to understand by means of a bodily organ; since the determinate nature of that organ would impede knowledge of all bodies; as when a certain determinate color is not only in the pupil of the eye, but also in a glass vase, the liquid in the vase seems to be of that same color.
We may understand this argument like this: the human eye is able to see because it is somehow light-like. The Ear can hear because it is sound-like, and so on. But the ear cannot see and the eye cannot hear. A physical organ able to understand one thing, by this logic, would be incapable to understand any other. From this, Aquinas concludes that the intellect cannot be something material. At the core of this argument is the idea that the ability of the senses to sense a certain type of thing must be specific for only this one thing, by its nature, “because that which is in it naturally would impede the knowledge of anything else. […] Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies. Now every body has its own determinate nature. Therefore it is impossible for the intellectual principle to be a body.”
I imagine Aquinas, a quill in his hand, scribbling these thoughts on a piece of parchment. I imagine him dipping his quill into the inkwell. Some ink is sucked into the shaft of the quill. Then the tip of the quill is scratching over the parchment’s surface, giving the ink flowing out of it a new form, the form of letters “Omne autem corpus habet aliquam naturam determinatam.” All bodies have some determinate nature.
The ink, without difficulty, is taking the form of the words representing Aquinas’s thought. Just like human understanding is able to take arbitrary thoughts as its form, the ink is able to take the form representing the philosophy of Thomas Aquinas. By the logic of his argument, the ink, being a material object with a certain nature, should have refused to take that form. Instead, like water on the surface of a Lotus leaf, it should have contracted into drops, taking its natural form and running down to its natural place. Alternatively, we would have to conclude that the ink – and perhaps the parchment – is an immaterial object. Aquinas, however, lost in his thought, did not look at the ink, he did not consciously see it flow from the quill and form itself into a diversity of words, drying and fixing itself into the words we can still read, downloaded from the internet.
I imagine Aquinas sprinkling fine sand over the parchment to suck up any excess ink, and blowing it away.
Postscriptum: Aquinas’s mistake was to think that if thinking is material, it must have a fixed form. Since it does not have a fixed form, it cannot be material. In a way, cognitive science steped into the same trap hundreds of years later: If thinking is material, it must have a fixed form. So let’s find that fixed form, the laws of thinking. Both where unable to imagine a material thing without a fixed form.
(The picture, a painting by Botticelli showing Thomas Aquinas, is from https://commons.wikimedia.org/wiki/File:Aquinat.jpg Painting is another example, besides ink, brains and computer memories, of material things able to take any form).
 That the translation I am using here translates “anima hominis” simply as “soul” reflects the later post-cartesian development where the concept of a soul was restricted to humans. With respect to the conceptual framework of Aquinas himself, this might be seen as an anachronism.
 For the English translation used here, see http://www.documentacatholicaomnia.eu/03d/1225-1274,_Thomas_Aquinas,_Summa_Theologiae_,_EN.pdf.
 The Latin text of this passage reads:
Respondeo dicendum quod necesse est dicere id quod est principium intellectualis operationis, quod dicimus animam hominis, esse quoddam principium incorporeum et subsistens. Manifestum est enim quod homo per intellectum cognoscere potest naturas omnium corporum. Quod autem potest cognoscere aliqua, oportet ut nihil eorum habeat in sua natura, quia illud quod inesset ei naturaliter impediret cognitionem aliorum; sicut videmus quod lingua infirmi quae infecta est cholerico et amaro humore, non potest percipere aliquid dulce, sed omnia videntur ei amara. Si igitur principium intellectuale haberet in se naturam alicuius corporis, non posset omnia corpora cognoscere. Omne autem corpus habet aliquam naturam determinatam. Impossibile est igitur quod principium intellectuale sit corpus. Et similiter impossibile est quod intelligat per organum corporeum, quia etiam natura determinata illius organi corporei prohiberet cognitionem omnium corporum; sicut si aliquis determinatus color sit non solum in pupilla, sed etiam in vase vitreo, liquor infusus eiusdem coloris videtur. Ipsum igitur intellectuale principium, quod dicitur mens vel intellectus, habet operationem per se, cui non communicat corpus. Nihil autem potest per se operari, nisi quod per se subsistit. Non enim est operari nisi entis in actu, unde eo modo aliquid operatur, quo est. Propter quod non dicimus quod calor calefacit, sed calidum. Relinquitur igitur animam humanam, quae dicitur intellectus vel mens, esse aliquid incorporeum et subsistens.