It might seem that the soul is some emergent property or function of matter arising out of a material cause:
1. If the soul is the form of the body, then the soul only exists insofar as it informs the body. Therefore, it is reasonable to infer that the soul is something dependent on the body.
2. If souls are identified according to their activity (as a vegetative soul vegetates and a sensible soul senses) then we ought to consider them precisely to be activities and nothing more. For what we observe in living things are not souls as such, but vegetation and sensation, by which we infer that there is something other than what we observe which performs those activities. But this is an unnecessary abstraction, as we perceive nothing other than activity in a material body.
3. If the soul is the principle of movement in material things, then it interacts with material. But what is immaterial is impassible and cannot interact with what is material. Therefore the soul must not be immaterial but something material.
4. The end of a living thing is to preserve its life. But self-preservation is achieved in and through the preservation of the body. Therefore, life-as-end is not distinct from the body but is merely the integrity of the body and its functions.
Contra 1: If we regard the form as dependent on matter as a kind of supervening quality, then we would subvert the order of substance and accidents. What is accidental to a thing is secondary to what the thing primarily is, which is its substance. When the soul is called the form of the body, it is precisely meant as the substantial form. Even though matter is a necessary part of a natural living thing, the matter is not enough to identify that thing as what it is, as we see that a plant or animal can grow and diminish over time while maintaining its individuality. But to say that a living thing's soul exists only insofar as it has matter is to put accidents prior to substance or else make matter the substantial form of the body, which is absurd. Hence, if we wish to concede that there are individual living things, then the primacy of the soul is a necessary conclusion from what is perceived.
Contra 2: We ascertain the form of a thing when we identify what it is specifically (i.e. according to a species), for matter is informed according to its species by its form. This is not merely a logical device, for to refer to a substantial form is to say that there is some active principle in that living thing which unifies the parts into a whole. Now when we observe some activity in a certain thing, we can attribute that activity in two ways; to the part in which the activity is manifested, and to the whole to which the part belongs. E.g. when a worm is prodded, we may say that there is a reaction in certain material parts of that worm, and that what we observe is the action of material apparata in response to some material stimulus. This may be true with respect to chemistry, but it is ultimately unintelligible without respect to the thing acting, viz. the worm. If we cannot account for the substantial form of the worm, then we cannot say that the worm moves, but only that the parts of the worm move. But this is an unnecessary reduction, whereas it is reasonable to attribute the movement observed within a thing to the thing itself.
Contra 3: It would indeed seem that immaterial things cannot interact with material for two reasons; viz. that what is material cannot harm or hinder what is immaterial (e.g. we cannot tie down an angel or transplant a soul) and that there is no known mechanism which would explain how such an interaction could take place. But we should not form too simple a concept of immaterial things. Some tend to think of interaction in terms of bodies reacting to force or contact, like two rocks hitting together. But no interaction at all can be attributed to purely material causes. The rocks that one knocks together are being moved ultimately by something that does the moving. So the answer to the question, how does the immaterial interact with the material, is found in the question, “what is interaction?” Strictly speaking, “pure matter” does not act. To hold this would ultimately be to conflate material cause and efficient cause and to disregard the act-potency distinction altogether. Therefore we must concede that there is something immaterial in a living thing which is the principle of its action.
Contra 4: If the soul is the substantial form of a living thing, then the body is the matter which is informed by the soul, and by means of which the soul acts. Thus the integrity of the body is due to the soul's act, which works in and through the body as the means to preserve its proper place as form. It does not follow that what is attained by means of the body is the body itself.
