We have been discussing the view of Thomas Aquinas according to which (i) the soul is the form of the body, and (ii) the souls of some animals, namely rational animals, are subsistent, i.e. capable of an existence independent of matter. I have registered some of my misgivings. Here is another. If our souls are subsistent forms, then why are not the souls of non-human animals also subsistent? If that in us which thinks is a life-principle and the substantial form of our bodies, and subsistent to boot, by what principled means do we not ascribe subsistent souls to all living things or at least to many non-human living things?
In this passage, Socrates puts the following question to Theaetetus: ". . . which is more correct — to say that we see or hear with the eyes and with the ears, or through the eyes and through the ears?" Theatetus obligingly responds with through rather than with. Socrates approves of this response:
Yes, my boy, for no one can suppose that in each of us, as in a sort of Trojan horse, there are perched a number of unconnected senses which do not all meet in some one nature, the mind, or whatever we please to call it, of which they are the instruments, and with which through them we perceive the objects of sense. (Emphasis added, tr. Benjamin Jowett)
The issue here is the unity of consciousness in the synthesis of a manifold of sensory data. Long before Kant, and long before Leibniz, the Plato was well aware of the problem of the unity of consciousness. (It is not for nothing that Whitehead described Western philosophy as a series of footnotes to Plato.)
Sitting before a fire, I see the flames, feel the heat, smell the smoke, and hear the crackling of the logs. The sensory data are unified in one consciousness of a selfsame object. This unification does not take place in the eyes or in the ears or in the nostrils or in any other sense organ, and to say that it takes place in the brain is not a good answer. For the brain is a partite physical thing extended in space. If the unity of consciousness is identified with a portion of the brain, then the unity is destroyed. For no matter how small the portion of the brain, it has proper parts external to each other. Every portion of the brain, no matter how small, is a complex entity. But consciousness in the synthesis of a manifold is a simple unity. Hence the unity of consciousness cannot be understood along materialist lines.
This argument from the unity of consciousness, which of course needs to be more rigorously developed, is present in nuce in Plato in the passage cited. According to Aquinas, if this argument is sound, ". . . it follows that even the souls of brute animals are subsistent." This seems to be a correct inference.
Aquinas hopes to block the inference with the help of Aristotle's De Anima 429 a 24. Thomas gives an argument that I interpret as follows:
1. Although understanding alone is performed without a corporeal organ, as Aristotle maintains, sensation and the operations of the sensitive soul are accompanied by changes in the body at our sensory receptors.
Therefore
2. The sensitive soul has no per se operation of its own, and every operation of the sensitive soul belongs to the soul-body composite.
Therefore
3. The souls of brute animals, having no per se operations, are not subsistent.
Unfortunately, I cannot see that this is a good argument. The gist of the argument is that while there are specific corporeal organs for seeing, hearing, smelling, tasting, and touching, namely, eyes, ears, etc., there is no corporeal organ for understanding. From this it is concluded that there is a crucial difference between sensing and understanding: it is broadly logically possible that there be understanding without a body; but it is not broadly logically possible that there be sensing without a body. And since we alone among animals are rational, or capable of understanding, we alone have subsistent souls.
What this ignores is that sensing is not merely a mechanical or material process. When my cat simultaneously sees, smells, and tastes her food (or my food as she much prefers) there is a unity of consciousness in her no less than there is one in me when I lay into my food. There is no specific corporeal organ that does this unifying of sensory data, neither in me nor in my cat. The unity-of-consciousness argument against materialism can be 'run' both for man and cat. If it works for me it should work for her. So if the possibility of my disembodied existence follows from there being no physical organ that unifies my consciousness, then we get the same result for my cat, and the difference between man and brute in respect of subsistence of souls cannot be maintained.
To sum up. Thomas wants to say that men, but no brutes, have subsistent souls. This is because men, but no brutes, understand. But sensing is a form of consciousness, and consciousness cannot be understood in materialist terms. Sensing is not a mere collision of atoms in the void. Sensory consciousness, besides displaying unity across its several modalities, reveals qualia. And qualia are a well-known stumbling block to materialism. It is difficult to see why, if understanding supports the possibility of disembodied existence, sensing should not also support this possibility. There is after all only one soul which both senses and understands. The phrases 'sensitive soul' and 'intellective soul' are not to be taken to refer to distinct souls.