I think it would be useful to reinstate a philosophical term that has been around since ancient Greece, but nowadays has little but Theological usage. I want to give it something close to the meaning it had in ancient Greece: whatever it is that makes a body be alive.
The soul is the form of the unifying energy of a living body.
Since, as we have been saying above, the form of the body's unifying energy as such is an abstraction, then so is the soul; it is the soul with its quantitative limitation that is the "concrete" soul; and this, of course is the unifying energy of the living body.
So the soul is not the life of the body, precisely; it is the form of life of the body, since life is the existence when the existence is in control of itself and is not dominated by its quantity; and in the living body, the form of this existence is called a "soul."
Another way of thinking of "soul" is that it is a form of unifying energy if the energy controls itself (or is not controlled by its quantity).
Why bother with such a term? Because it is going to be inconvenient from here on to be talking about "the form of existence when the form is not under the control of its quantity." It is much easier to have a term which means that.
As to its preemption by Theology, what happened is that Plato's philosophy heavily influenced early philosophizing on Theological subjects; and Plato held that the soul was a "something" that got into a body and made it live (not a difficult mistake to make, as we have been seeing), and was basically spiritual. Christianity spoke of immortality and the human spirit (though Christianity talked of a restoration of the body, which the Greeks in the Areopagus ridiculed St. Paul for holding).
It was not surprising, then, that the focus of attention was on the spiritual human soul in Christian philosophy, even when St. Thomas adopted the Aristotelian notion (which is very close to mine, above) that the soul was the "substantial form" of the living body, and so was limited by matter. St. Thomas, of course, established that the human soul was spiritual, though "transcendentally related" to its matter.
Of course, Theologians aren't interested in souls of tomato plants or cockroaches, but in human souls; and so the term "soul" in ordinary usage nowadays refers to the spiritual soul that human beings have, and people look at you oddly when you tell them that toads have souls and so do cabbages.
It is actually because of the Theological usage of the term that biologists don't like to talk about "souls"; the term smacks too much of that mysterious something that gets into a body and directs it, and seems to imply the metempsychosis of souls that need punishment into lower forms of life, as Plato and various Indian philosophies held.
But actually, there is nothing in "soul" as I have defined it that a biologist couldn't accept, though he might be a little uncomfortable about the "freedom from control by its quantity" I spoke of. Biologists have no trouble admitting that living bodies are organized, and that the different kinds of organization account for the different kinds of living bodies; and the soul, after all, is just the way the living body is organized; and as such it is the kind of interaction that the parts have with each other by which they act together instead of independently, as in the corpse.
So in that respect, any biologist would admit that living bodies have souls. Where the problem comes is, as I say, in this self-control and freedom from quantity that we concluded to. And the reason for this is that, given the focus the biologist has (What mechanism does the body use for X? How does this mechanism work?), they are not attuned to the implications of the living acts for the way the body as a whole is organized.
But this shouldn't give them any problem, because they should be able to follow the reasoning we have given up to this point. Unfortunately, many of them are not only not interested in doing so, but think that our kind of investigation is a sham. It is here that biologists turn into biologians, and make a religion of their own focus on things; and instead of seeing problems that don't pertain to that focus, they say that such things are pseudo-problems, because after all they are the scientists that deal with living bodies, and everybody knows that philosophy is a branch of astrology. The result is that serious problems are given simplistic "solutions," because the problem is not really faced. That is, everyone (but another biologian, of course) who casts doubt on the mechanism of evolution that they talk about is automatically a "creation scientist" who is trying to prove that things happened in seven days a few thousand years ago.
But that is enough, I think, to get the spleen out of my system. Suffice it here that I think that "soul" is a useful term to have, but that it should be properly understood as just the form of the unifying energy when the energy is not controlled by its quantity.
But since I have a term that talks about the form of unification of a body when that body is a living one, I am also going to use the term "soul" in a slightly less strict sense than just the form of the unifying energy (i.e. as just the limitation of existence), and so in itself nothing at all.
Soul will often be used in a looser sense as meaning life as limited in the way in question.
That is, "soul" in this sense includes the existence and means "existence in this form" when the existence can be called "life" and the life is the life of a body.
One final remark to make about the definition. Only bodies have souls (because it is the form of unifying energy of a body). Pure spirits are forms of life (and so is a soul) but pure spirits are not souls, because these forms of life do not organize bodies. The human soul organizes a body; but after death it no longer does so (as we will see). Is it a soul then? Yes, because it is a form of life which by nature organizes a body, and the disembodied human soul is in an unnatural condition, as we will see later. St. Thomas says that the disembodied soul after death is still "transcendentally related" to the body it used to have (which differentiates each of us after death and makes us not be absorbed into a kind of "humanity as such"); but we will also see this later, because my version of what means is somewhat different from what I think St. Thomas meant by it.Next