Chapter I -- 3
In the above discussion, we have dealt with St Gregory’s first case, that the soul might be in the elements of the body. St Gregory now has a second case. This is to be gleaned from continuing to quote from his argument, at the point where we stopped to discuss the first case:
For there would not occur a certain commingling of that which is of a different nature with that which is alien, and if these things should be, that which has been commingled with the opposite qualities will be manifested plainly and completely as being to an extent complex.
The first part of this sentence has this meaning: For the first case, the one wherein the soul is in the elements of the body, St Gregory has asserted that the soul would necessarily then be the same thing as, or be one with, those elements. As his reason, he asserts that something different in nature would not mix with the elements of the body, since the elements would be alien to it. Such a something would be some sort of non-material substance.
In the second part of the sentence, he introduces his second case: If, however, such a thing should come to pass, that some sort of non-material substance should be joined to the elements of the body, and that non-material substance should be the soul, then that soul must necessarily be complex (or, ‘various’). Evidently, St Gregory’s logic is this: if we suppose that a non-material substance, call it the soul, should be joined to the elements of the body, wherein we observe in the various parts of the body contrary qualities, then that non-material substance would itself have to have a certain complexity. This is evidently because it would have to be united to parts of the body which have contrary qualities. The doctrine of contrary qualities, certainly Platonic and Aristotelian, is connected to the physics and chemistry current in St Gregory’s day: we have pairs of contraries such as hot and cold, dry and moist, each of which characterizes the parts of the body on the basis of which element of the four, earth, air, fire and water, predominates. We would say today that the chemical elements, and, perforce, the molecules in the body which contain them, have various chemical properties. To be able to latch on to all the parts of the body—all the chemical compounds—with their various chemical properties, St Gregory is saying, the non-material substance, call it the soul, would certainly have to be somewhat complex. It could not be simple. This is the key to his argument. His logic, difficult to grasp, has this sense: The chemical compounds in the body have different chemical properties, some of them opposite in quality: some are acids, some bases; some are hydrophilic, some hydrophobic; and so on. Any substance—whether non-material or material, although it does seem that St Gregory has in mind a non-material substance; what he says is something of a different nature and alien from the body—any substance, then, which were to connect into the body would necessarily connect into acids and bases, hydrophilic and hydrophobic compounds, and so on. To be able to make all the necessary connections at all points of the body, that substance of a different nature from and alien to the compounds of the body would itself have to be somewhat complex, because the compounds of the body have different and opposing chemical, and, more generally, physical characteristics, and that non-material substance would have to connect into them all. A simple substance different from the nature of the compounds of the body, because it would be simple—the same all over, not decomposable into smaller units or faces—would not be able to make contact with all the chemical compounds because of their complex and varying or even opposed chemical characteristics.
The question arises, although St Gregory does not discuss it, given that electromagnetic waves were discovered 1500 years after his death: well, could the soul be some sort of electromagnetic field?
An electromagnetic field has a certain complexity; its mathematical description foresees that certain scalars or vectors or tensors will have different values at different points in space and time. Neither is an electromagnetic field simple. Hence, even if one were to think in these terms—and we do not for the reason that a field of electromagnetism or of any other force known to science is a physical phenomenon—then the localization of the soul, whatever it might be, in such a field would founder on what St Gregory is going to say next.
For St Gregory goes on that the complex is not simple, but always seen in the composite. But what is composite is necessarily susceptible of dissolution. This is clear to us even today: molecules once made can be taken apart; electromagnetic and other force fields change, diminish and disappear. And that dissolution is the decay of what was constituted, the molecule or force field in our examples: a large molecule, say haemoglobin, when it dissolves loses its identity as haemoglobin; a force field once diminished is gone as a force field in the region. But what decays is not immortal. Recall that St Gregory is concentrating on arguments against the survival of the soul after death. What he is saying is that once we accept his argument that the non-material substance, call it the soul, in order to latch on to all the parts of the body must be complex, then we must concede that it must be susceptible to dissolution and decay, and, if so, we must concede that the soul cannot be immortal because it can decay into its constituent parts. Otherwise, the flesh itself would have to be considered immortal, which is manifestly absurd.
Let us repeat the steps of St Gregory’s argument, which are difficult. He has said in his first case that the soul must be in the elements of the body—we would say, a part of the body or the functioning of a part of the body or the DNA or even the information content of the DNA—since if it were of a different nature from the elements of the body then it could not mix with the elements of the body. Then, as his second case, he says: well even if I concede that the soul is of a different nature from the body but still mixed with it, then the soul must be complex. His reason is that if the soul were of a different nature from the body, then it would have to be somewhat complex in order to attach itself to all the various material parts of the body at the biochemical level. But if the soul is complex, it is composite; if it is composite, it can be decomposed; if it should be decomposed it will have lost its identity as a whole; if it should lose its identity as a whole, it will no longer exist as a whole; and if it is capable of no longer existing as a whole, it is not immortal of necessity.
How good is this argument? We find difficulty in accepting St Gregory’s step that the soul must be somewhat complex in order to latch on to the various parts of the body at the biochemical level. We are not persuaded. Once one accepts this step, however, the rest of the argument follows. The assertion that something capable of decaying and losing its identity as a whole is necessarily not immortal requires comment, however. This is an Aristotelian position, the so-called ‘Principle of Plenitude’: ‘No genuine possibility can remain forever unrealized.’
This argument against the survival of the soul after death, quite abstract, does not have any adherents today as far as we know, although it quite possibly could be advanced by a materialist-mechanist against the possibility of a non-material ‘ghost’ in the body, the machine, that survived the death of the body. It is an important but theoretical argument against the existence of a soul which is different in nature from the body but which survives the death of the body.
It appears, however, that the basis of St Gregory’s introduction of this argument is the analysis of the nature of the soul by Epicurus.
It seems that Epicurus treated the soul as a sort of ‘subtle body’ inhabiting the body that we know, but composed of a different sort of substance—call it a ‘subtle’ substance—than the body. In Epicurus’ view, which we will discuss later in detail, this subtle body dissipated after death. St Gregory’s argument then becomes clear as a presentation, in anonymous form, of Epicurus’ reasoning why the soul cannot survive death.
 We would say, today, in the various chemical compounds.
 For a discussion of this principle in the context of the philosophy of the High Middle Ages, see S. Knuuttila’s Chapter 17, ‘Modal Logic’, Section: ‘The principle of plenitude’, in the Cambridge History of Later Medieval Philosophy (Medieval pp. 344 ff.). See also, from a Platonic point of view, the Phaedo (Plato Volume I, 78c1–8).