For Thomas Aquinas, (i) the soul is the form of the body, anima forma corporis, and (ii) the souls of some animals, namely rational animals, are subsistent, i.e., capable of existing independently of matter. But he denies that the souls of brutes are subsistent. So after we die, our souls continue to exist as subsistent forms, but the souls of brutes do not.
The question arises: If our souls are subsistent forms, then why are the souls of non-human animals not also subsistent? If the soul of a rational animal is its life-principle, and the substantial form of its body, and subsistent to boot, why aren’t the life-principles and substantial forms of non-rational animals also subsistent?
Aquinas discusses this question in Summa Theologica, Q. 75, art. 3. He attempts to navigate a middle course between a form of materialism according to which sense and intellect are both "referred to a corporeal principle," and the Platonic view according to which both sense and intellect are "referred to an incorporeal principle." This gives us three views to consider.
Materialism: both sensory and intellectual awareness/knowledge are material in nature. There is no subsistence of the soul in man nor brute when the body dies.
Platonism: Both sensory and intellectual awareness/knowledge are immaterial in nature. The soul survives in both man and brute when the body dies.
Thomism: Intellectual awareness/knowledge alone is immaterial in nature. Only in rational animals is there such awareness/knowledge. So, there is survival of the soul in man alone when the body dies.
Setting materialism aside, the fight is between Plato and Thomas.
On the Platonic view, both sensing and understanding "belong to the soul as such." Thomas refers us to Theaetetus 184c. This is a fascinating passage which, according to the Paul Natorp, anticipates Kant's transcendental unity of apperception, as indeed it does.
At 184c 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?" Theaetetus 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, Plato was well aware of the problem of the unity of consciousness. (It is not for nothing that Alfred North Whitehead described Western philosophy as a series of footnotes to Plato.) But what exactly is the problem?
Sitting before a fire, I see the flames, feel the heat, smell the smoke, and hear the crackling of the logs. The sensory data from four different outer senses are somehow unified in one consciousness of a synchronically and diachronically selfsame object. E pluribus unum! No one object would appear to me at a time or over time were it not for the ongoing synthesis in one consciousness of the various sensory givens. This unification of data 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 tenable answer.
For the brain is just a marvelously complex hunk of meat. It is a partite physical thing extended in space. If the unity of consciousness is identified with the brain or 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 partite entity. But the consciousness of the object via the synthesis of a manifold of sensory data is a simple (non-complex) unity. Hence the unity of my consciousness of the fire 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 Thomas cites (Theaetetus 184 c). Thomas, however, misconstrues Plato’s argument as the one he presents in Objection 2:
a) The sensitive power of the soul is to sensible objects as the intellectual power is to intelligible objects.
b) The intellect without the body apprehends intelligible objects.
Therefore
c) The sensitive power without the body perceives sensible objects.
According to Aquinas, if this argument is sound, ". . . it follows that even the souls of brute animals are subsistent." This is a correct inference on Aquinas’ part.
Aquinas rejects the above argument by rejecting premise (a) with the help of Aristotle's De Anima 429a24. Thomas argues against (a) 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. Sensory data arise only if bodily sense organs are affected by external things.
Therefore
3. The souls of brute animals, since they are merely sensitive and thus lack per se operations of their own, are not subsistent.
So, when I die, I continue to exist as a subsistent form; my cat, however, does not similarly survive his bodily death. I can look forward to the beatific vision and the resurrection of the body; he can’t.
I grant that Thomas has refuted the argument he presents in Objection 2. I deny, however, that the refuted argument is Plato’s unity-of-consciousness argument. What Aquinas ignores is that sensing is not merely a mechanical or material process whereby bodily sense organs are affected by physically external things causing changes in the organs. He falsely presupposes that it suffices for sensory knowledge of an object that sense organs be affected by external things. Affection, however, is necessary but not sufficient for sensory knowledge of an object. This is because the data from the different senses cannot amount to an awareness of one and the same object unless they are unified or synthesized. Disparate data merely in consciousness would lack of-ness or object-directedness (intentionality) were there not some synthesizing ‘principle’ that assembled the disparate data in such a way as to give them reference to an external thing. This ‘principle’ cannot be corporeal or material.
When my cat simultaneously sees, smells, and tastes his food there is a unity of consciousness in him no less than there is one in me when I enjoy my food. There is no specific corporeal organ that does this unifying of sensory data, either in me or in my cat. The unity-of-consciousness argument against materialism can be 'run' both for man and for cat. If it works for me it should work for him. So if the possibility of my disembodied existence follows from there being no physical organ that unifies my conscious data so as to give them objective reference, 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 not brutes, have subsistent souls. This is because men, but not brutes, understand and perform purely intellectual operations. While this is true, it does not give Aquinas the conclusion he wants, namely, that men, but not brutes, have subsistent souls. For it is also true that sensory awareness of a physical thing is not itself physical (material). Sensing is a form of object-directed consciousness, consciousness-of, and consciousness-of cannot be understood in materialist terms. Sensing, whether by man or brute, as when either I or my cat senses an approaching dog, for example, is the presentation, at a time and over time, of a selfsame object together with its properties. But it is not the sense organs as recipients of causal impacts from external things that can effect this presenting of a unified object. What they deliver, at most, is a manifold of data that neither cohere among themselves, either synchronically or diachronically, nor refer to a selfsame object, the dog in our example. So there is need for a synthesizing or unifying ‘principle,’ which, we have seen, cannot be material due to the partite and thus non-simple nature of matter. The synthetic unity of consciousness cannot be understood in terms of axons, dendrites, the diffusion of sodium ions across synaptic junctions, etc. It is not a hunk of brain-meat or any part thereof that knows anything whether sensorily or intellectually.
What does the synthesizing ‘principle’ operate upon in its constitution (Husserl’s term) of the intentional object (the object as it appears, and just as it appears) to the conscious subject? It operates upon the sensory data, the qualia, which, on a realist scheme such as that of Aquinas, occur in consciousness as a result of the causal impacts of external things upon our material sensory receptors (eardrums, etc.). These qualia are also a well-known stumbling block to materialism. (How is it possible that the smell of cooked onions, or the burning sensation consequent upon chowing down on Habanero chiles, be identical to anything in the brain or in anything physical anywhere?)
I conclude that there are two reasons why sensation cannot be be “referred to a corporeal principle.” Sensory awareness/knowledge of objects, of the Habanero chile on my plate, for example, cannot be so referred because there is need of a non-corporeal synthesizing ‘principle’ to unify the sensory data so as to make them refer to one and the same object, and the sensory data themselves cannot be so referred because they are manifestly non-identical to anything physical.
It is therefore difficult to see why, if understanding supports the possibility of disembodied existence, sensing should not also support this possibility. If this is right, then not only will I survive my bodily death as a subsistent form, but my cat Max will as well when it is his turn to slough off the mortal coil.