If you were asked whether or not you are alive, the fact that you can understand and respond to the question would be a sound basis for an affirmative answer. To be alive is to be able to hear, see, feel, think, want, dream, move about, grow, and the like. This suggests levels and degrees of vital operations.
Before they send you off to the morgue, they will check you for some of the more basic vital signs we listed. And some more sophisticated ones as well, murmuring about "brain death." In keeping with our preferred procedure -- it is at once Aristotelian and natural -- we will be content with obvious instances.
The mirror held to the mouth of the ailing person is meant to catch a sign of life. If it clouds, there is breath, and where there is still breath we have not yet inherited. Breath. Wind. The Greeks took their word for the principle of life from that. Psyche. It becomes anima and spiritus and in English Soul.
The associations of words with psych- as their prefix, to say nothing of the movie Psycho, are unhelpful, and "soul" will have for most of us religious overtones. The soul is what Jesus saves, as Scripture and a host of highway signs assure us. Psychic research will suggest the slightly wacky, and talk of psychological phenomena seems meant to whisk us off to some strange and separate conceptual landscape. In reaction to that, many spend their lives proving that life is no different from other natural events. That is a healthy if mistaken conviction.
St. Thomas in his theological writings makes use of what Aristotle, and others, have had to say about the human soul, but such passages may not convey well the philosophical setting in which talk of the soul arises. That context is natural philosophy, that is, the effort to understand the sensible things around us. A first analysis of them as natural or physical things led to the realization that, as products of change, they are complex, a subject and a form. That is true of all natural things, though admittedly it does not tell us much about any of them specifically, however concrete and accessible the examples. Some natural things are living, others are not.
How do we know that? How do we know we are alive? Thomas dos not hesitate to say that we experience in ourselves that we have a soul and that it is by the soul that we live. A soul? It is easy to imagine objectors trying to test the floor to deny that they have any such experience. They are not aware of any soul within themselves, some mysterious engine, utterly unlike any other natural thing. . ..
We cut off the microphone and with it the threat of profanity. Such objections are stimulated by many things, none of them particularly relevant to the current effort. I am tempted to suggest that we forget for the moment everything we have ever heard of spirit and soul and psyche. Think instead of breath. Breathing is a sign of life. A hard tackle can knock the wind out of an opponent and immobilize him. Announcers speak in hushed tones until they can say, "I saw him move." If what is seen is not a final twitch, we all conclude that the split end will live to catch another pass. The trainer will ask the recovered player questions to see if he is all right. His ability to say where he is, like breathing and movement, is taken to be a sign of life. Some of these signs are intermittent, and we do not consider something that has in the past and will be in the future talk but cannot at the moment to be no longer among the living.
Soul is the principle of vital activities.
That's all that it is for our present purposes. Furthermore, the difference between living and nonliving natural things is a deep one. It is not superficial, like being heavy or light, here or there, large or small. To be alive is not accidental to a subject or substance. Some substances are essentially living, others not.
But can't we say that some natural things happen to be alive and others happen not to be? We just did. But when we do we don't mean that Socrates or a horse or a carrot are living things because there is a substance which from not being Socrates now happens to be Socrates, a substance which from not being a horse now happens to be a horse, and so with carrot. Why don't we mean that?
When Socrates comes to be, a new substance comes into existence. When Socrates ceases to be, a substance goes out of existence. Socrates as such is a living thing. So when he comes to be, a living substance comes to be. A dead horse is not a horse in a new state of affairs. What is referred to as a horse is no longer a horse. If someone sold him to you you would not agree you had purchased an odd sort of horse. What you have is no horse at all. It is no longer a substantial unit. Absent its principle of life, it will rapidly reveal that it is one only as a pile of things is one.
That is what we mean by denying that to be alive is accidental to a substance. There is no same thing, no permanent substantial unit, which is sometimes alive and sometimes not. The living thing is a substantial unit.
It all comes down to being able to identify some natural or physical objects as alive and others as not alive. Educated as we have been, we will have been told of species which seem difficult to classify as animals or plants, and plants which seem to fade away into the realm of the nonliving. Is crystal formation a sign of life? How very obscure it can all become, and so quickly.
Indeed it can. All the more reason to cling to the certain, to Thomas's remark that each of us has the experience of life and that the soul is that whereby he lives.
Is Thomas referring to some private and incommunicable experience, some introspective certainty over which we could quarrel endlessly? Why not put it on the basis of objective, observed happenings? But then we are going to confront puzzles about viruses and crystals and the like. As Charles DeKoninck remarked, the beauty of St. Thomas's base is that it combines the inner and the outer.
Having been struck by a Mack truck, I am carried to the hospital where I come hazily back to consciousness and hear the anxious voices of my heirs asking if I am still among the living. I lift a hand and waggle my fingers, casting gloom over the assembly. The man is still alive.
What this gathering of eagles sees is a distinguished and despite his injuries still prepossessing old gentleman waggle his fingers. An objective experience, in the jargon we are accepting. From my supine vantage point I also observe my waggling fingers but I also am aware that I am causing that movement. Not only do I see the vital movement, I sense that I bring it about. That is when I cannot fail to know with certainty that I am alive. When I see similar activities in others I reasonably conclude that they too are alive, but it may well be talking about it provides the surest bridge. Our conviction that there are other minds resides on the firm foundation of waggling fingers, breathing, pushing when shoved, and the like.
The soul is that whereby we primarily live, perceive, and think—primarily because of what we said above about living substances. To be alive is not an accident of a substance. Soul cannot be an accidental form. It is, then, a substantial form.
We can now link up with things said earlier. When a natural substance comes to be, a subject which is not itself a substance, prime matter, receives a new form. If the subject were a substance, the new form would be an accidental form. The substance is that to which the change is attributed and which survives the change. Such a substance would happen to be alive as it happens to be tan.
This cannot be because when the living thing Socrates comes to be, a new substance comes to be, and when he dies, a substance is no more. The soul then is the substantial form of the living thing.
It is important to see that soul is a special kind of substantial form and that knowledge of it emerges from an analysis of change. We are in the realm of natural philosophy.
But what does it mean to say that knowledge of it arises from an analysis of change, if as we spent some time asserting there is an immediate and irrefragable certainty of life? Certainty is not always clarity. That I am sure I am alive is as true as can be, but this does not mean I know what life is. The analysis mentioned is a first step on the way to understanding what life is. We were certain of being alive long before we had any theory about life and, to repeat an earlier point, our certainty survives the theory, or so much the worse for the theory. It is not in the cards that we will end up saying there is no difference between what we initially called living and what we initially called nonliving. The theory is meant to account for this difference, not eradicate it.
Neither our original certainty nor these first accounts of the soul -- as first principle of vital activities, as substantial form of an organized body -- make any claims at all about what we may call the spirituality of the soul.
That the soul is immaterial follows from the fact that it is a form, and in that sense all substantial forms are immaterial -- are forms, that is, and not matter. It is all but impossible to think of the human soul without being reminded of philosophical -- think especially of Plato -- and religious doctrines which hold that our soul preexisted its union with body or will survive death or both. Nothing like this is being claimed or assumed at the outset. That inner experience of life is not the experience of a spiritual thing, that is, of a thing that can exist apart from the body; What one is aware of is vital activities and that there is within us a capacity to perform them. If any of those activities or the soul itself turn out to be dramatically different from other natural activities or substantial forms, that will have to be established by argument. It is not, again, the assumption of the discussion. At this point, materialists and nonmaterialists are one big happy family.
[213] Note that, as the Philosopher teaches in Metaphysics VII, there is this difference between the definition of substance and accident, that nothing is put into the definition of substance that is outside the substance being defined, since a substance is defined by its material and formal principles. But in the definition of an accident is put something outside the essence of the defined, namely, its subject: the accident's subject must enter into its definition. E.g., when we say "snubness is the curvature of the nose." The reason for this is that the definition signifies what a thing is: but substance is something complete in its existence and kind, whereas accident does not have complete existence, but is dependent on substance. Similarly no form is something complete in kind: that completeness of kind belongs to the composed substance. Therefore the composed substance is so defined that nothing outside its essence enters into the definition. But in the definition of form, something outside the essence of form is mentioned, namely its proper subject or its matter. Hence, since soul is a form, it is necessary that its subject or matter enter into its definition.
[214] So in the first place, he sets down two divisions, the first of which is necessary for investigating what is put into the definition of soul to express its essence, the second, what is necessary to investigate what is put in the definition of soul to express its subject, there, "substantiae autem maxime, etc." In pursuit of the first he sets down three divisions, of which the first is that according to which being is divided into the ten categories. This he implies when he says that substance is one kind of being.
[215] A second division is according as substance is divided into matter and form and composite. Matter indeed is that which in itself is not a definite thing (hoc aliquid). Form is that thanks to which the thing is now actual. The composed substance is a definite thing. That is called a definite thing (hoc aliquid) which can be pointed out as complete in being and kind and, among material things, this is true only of the composed substance. For separate substances, though not composed of matter and form, are definite things since they actually subsist and are complete in their nature. The rational soul can be called a definite thing in a way, insofar as it can subsist by itself. But because it is not a complete species but is rather part of a species it cannot be said in every way to be a definite thing. The difference between matter and form is this: that matter is potential being, and form is entelechy, that is the act whereby the matter comes to be actually, such that the composite being actually is.
[217] He sets down divisions needed to look into what is put into the definition of soul as pertaining to its subject. And he suggests three divisions, the first of which is a distinction between substances which are bodies and those which are not. Bodily substances are the most obvious kind since incorporeal substances, whatever they might be, are not manifest to us, as not falling within the range of sensation but investigable by reason alone. That is why he says "bodies especially seem to be substances."
[218] A second division is between natural or physical bodies and artificial bodies. Man and wood and stone are natural bodies, whereas house and hatchet are artificial. Natural bodies seem more obviously substances than artificial ones because natural bodies are the principles of artificial bodies. Art works on matter provided by nature and the form imposed by art is an accidental form, just as shape and the like are. Thus, artificial bodies are not in the genus of substance because of their form but only because of their matter, which is natural. Their status as substance derives from natural bodies. That is why natural bodies are substances more than artificial bodies are because they are substances not only because of their matter but also because of their form.
[219] A third division is between natural bodies which have life and those which do not. That is said to have life which by itself has nutriment, growth, and decrease. This is by way of illustration rather than definition since a thing is said to have life not only because of nutriment, growth, and decrease, but also because it senses and understands and can exercise other works of life. Hence in separate substances there is life because they have intellect and will, as is said in Metaphysics XII, though there is no growth or taking food with them. But because in generable and corruptible things the soul which is in plants, to which nutriment and growth pertain, is the beginning of life, and that is why by way of example he explains "having life" by nutriment and growth. The proper understanding of life is that something is capable of moving itself, taking move in a wide sense such that even intellectual activity can be called motion. We say things which are moved only by an external principle are without life.
[220] He looks into the definition of soul on the basis of the foregoing distinctions. He does three things. First, he looks into the parts of the definition; second, he gives the definition; third, he responds to a difficulty. The first point is sub-divided into two: first he examines the parts of the definition pertaining to the essence of the soul, second those which pertain to the essence of its subject. The first is further subdivided, for first he looks into this part, that the soul is an act, and then that it is first act.
From the foregoing, then, he concludes that, since physical bodies are especially substances, and every body having life is a physical body, it must be said that every body having life is a substance. And since it is an actual being, it is necessary that it be a composed substance. Because when I say 'a body having life,' I say two things, namely that it is a body and a body of a certain kind, one having life. It could not be said that the part of the body having life which is called body is the soul. For by soul we mean that whereby the thing having life lives; so it must be understood as existing in a subject, taking subject here in a broad sense according to which not only what actually exists is called a subject, as an accident is said to be in a subject, but prime matter too is called a subject. The body which receives life is rather the subject or matter than what exists in a subject.
[221] Thus since substance is threefold, namely, the composite, matter and form, and soul is not the composite 'body having life' nor the matter, that is, the body which is the subject of life, it remains that soul is a substance as the form or species of such a body, namely, of a physical body having life in potency.
-- Commentary on the De Anima, Book Two, lesson 1