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This may be proved in three ways. The first…
Our intellects are not material, i.e not bodies. The proof of this is this week. The enormous consequences of this amazing fact have to wait until later.
Chapter 49 The the intellectual substance is not a body(alternate translation)
1 FROM the foregoing it is shown that no intellectual substance is a body.
2 For no body is found to contain anything except by quantitative commensuration: wherefore also if a thing contain a whole thing in the whole of itself, each part will contain a part, the greater part a greater part, and the lesser part a lesser part. But an intellect does not contain a thing understood by quantitative commensuration: because by its whole self it understands and comprehends both whole and part, things both great and small in quantity. Therefore no intelligent substance is a body.
Notes You can’t chop your intellect into pieces. Your arms, legs, nerves, and, yes, your brain, but not your intellect.
3 Moreover. No body can receive the substantial form of another body, unless it lose its own form by corruption. But an intellect is not corrupted, but rather is it perfected by receiving the forms of all bodies; since it is perfected by understanding, and understands by having in itself the forms of things understood. Therefore no intellectual substance is a body.
Notes Forms, don’t forget, are not material. For instance, there is no material form “as tray” in a clay ash tray, but there is clay.
4 Further. The principle of distinction between individuals of the same species is the division of matter in respect of quantity: because the form of this fire differs not from the form of that fire, except by the fact of its being in different parts into which matter is divided; nor is this otherwise than by division of quantity, without which substance is indivisible. Now that which is received into a body, is received into it according to quantitative division. Therefore a form is not received into a body, except as individualized. If, therefore, an intellect were a body, the intelligible forms of things would not be received into it except as individualized. But the intellect understands things by their forms which it has at its disposal. Consequently the intellect would not understand universals but only particulars. Now this is clearly false. Therefore no intellect is a body.
Notes Thus, contra some science fiction stories I vaguely remember, you can’t eat somebody’s brain and assimilate their intellect.
5 Again. Nothing acts except in accordance with its species, because the form is the principle of action in everything. If, therefore, an intellect be a body, its action will not transcend the order of bodies. Wherefore it would understand nothing but bodies. Now this is clearly false: since we understand many things that are not bodies. Therefore the intellect is not a body.
Notes Question: What’s 1/0? Answer: Not a body. (Joke.)
6 Again. If an intelligent substance is a body, it is either finite or infinite. Now, it is impossible for a body to be infinite actually, as is proved in the Physics. Therefore it is a finite body, if we suppose it to be a body at all. But this is impossible, since in no body can there be infinite power, as we have proved above. Now the power of the intellect in understanding is in a manner infinite, for by adding it understands species of numbers to infinitude, and likewise species of figures and proportions. Moreover it knows the universal, which is virtually infinite in its compass, since it contains individuals which are potentially infinite. Therefore the intellect is not a body.
Notes How can we know that 1, 2, 3, … goes to infinity? How can we know that (my old saw) that for natural numbers x and y, that if x = y then y = x for x, y = 1, 2, 3, …? How can we grasp any infinite concept? This is what induction is about. This is wonderful subject, to be explored later. For now, it is enough to concede our intellects somehow operate in an infinite manner.
7 Moreover. It is impossible for two bodies to contain one another, since the container exceeds the contained. Yet two intellects contain and comprehend one another, when one understands the other. Therefore the intellect is not a body.
8 Again. No body’s action reflects on the agent: for it is proved in the Physics, that no body is moved by itself except in respect of a part, so that, namely, one of its parts be mover and the other moved. Now the intellect by its action reflects on itself, for it understands itself not only as to a part, but as to the whole. Therefore it is not a body.
Notes Ain’t those last two arguments pretty, as Captain Aubrey would say?
9 Again. A body’s action is not the object of that body’s action, nor is its movement the object of its movement, as proved in the Physics. But the action of the intellect is the object of its action: for just as the intellect understands a thing, so does it understand that it understands, and so on indefinitely. Therefore an intellectual substance is not a body.
10 Hence it is that Holy Writ calls intellectual substances spirits: in which way it is wont to name God Who is incorporeal, according to Jo. iv. 24, God is a spirit. And it is said (Wis. vii. 22, 23): For in her, namely Divine Wisdom, is the spirit of understanding,…containing all intelligible spirits.
11 Hereby is excluded the error of the early natural philosophers, who held that there was none but corporeal substance: wherefore they said that even the soul is a body, either fire, air, or water, or something of the kind. Which opinion some have endeavoured to introduce into the Christian faith, by saying that the soul is the effigy of a body, like a body outwardly imitated.
Notes It’s not only the early natural philosophers who make this error, many modern-day natural philosophers, i.e. scientists, make it, too. Hence we see experiments where a live body is weighed and weighed again after death to determine the weight or mass of the soul. Or we see others, usually on television, where researchers look for “energy patterns” of the soul. And so on.