Chapter 075 - Of Man as He is Composed of Spiritual and Corporeal Substance; And First, as to the Essence of the Soul

The soul being the first principle of life cannot be a body. Body may, indeed, be a principle of vital operation, as the heart in animals, but it is not the first; for if the body were the first principle of life all bodies would be living. Hence if a body be living, or a principle of life, inasmuch as it is such, and so actuated it must of necessity have received this from some principle which is called its actuality. Therefore the soul is the actuality of the body, as heat, which is the principle of warming, is not a body, but the actuality or quality of a body.

This soul is self-subsistent and incorporeal; for that which takes cognizance of all bodies can by no means have the nature of a body. For if it had, one body would hinder the knowledge of another, as we see in the case of a tongue, bitter through indisposition, which can taste nothing sweet, but everything appears bitter to it. Therefore if the intellectual principle had the nature of any body in particular it could not know all bodies; for every body has some determinate nature. In the same way it follows that, if the soul understood by means of any corporeal organ, the determinate nature of such an organ would prevent the apprehension of all bodies; as we see in the case of a liquid poured into a glass, which appears to be of the same colour as the glass. Therefore the intellect must have an operation of its own, to which the body does not contribute; and as nothing can act by itself unless it be self-subsistent, it remains that the human soul is incorporeal and self-subsistent.

The sensitive soul, indeed, has no proper operation; all its operations are in union with the body, nor do they take place without some bodily change: the understanding only requires no corporeal organ; therefore the souls of the lower animals are not self-subsistent.

The soul, however, is not the man; for that is man which performs the works of a man; as anything is that which has the work proper to that thing. To feel, is not the work of the soul alone; and since feeling is a human operation, not proper to the soul alone but only when it is joined to the body, it follows that the soul is not, properly speaking, the man, unless it be understood as in some sense as the principle; as a city may be said to do what the ruler does.

Neither is the soul composed of matter and form; because being itself the form of the body it must be either all form, or partly so. If the former, it is impossible that it should be compound, because what is in potentia is not form; for form is actual; if the latter, we should have to call one part soul, and the other the first animated.

The same may be proved from the fact that everything is received according to the nature of the recipient; and the intellectual soul knows, universally and immaterially, things absolute in their nature - as, for instance, a stone - not this or that stone; whereas, if it were composed, the forms of things would be received individually, as is the case with the sensitive faculties.

Nor is the human soul corruptible, for no self-subsistent form is such, either in itself or by accident: having subsistence in itself it cannot be dissolved by accident; for a thing can only be dissolved or generated according to the mode of its being.

Moreover, corruptible things have a being separable from their constituents; but not so subsistent forms, since being cannot be separated from form. Hence a self-subsistent form cannot cease to be, even if it be allowed to be composed of matter and form; for corruption is only found when there are contrary forces; and celestial bodies are incorruptible because they contain no elements of contrariety.

In the intellectual soul, indeed, there can be no contrarieties, because it receives everything according to its own mode of being; and since the knowledge of contraries is one, it is evident that the reasons of contraries are received by the intellect without contrariety; therefore it is impossible that the soul should be corruptible. And we see an indication that this is so in the fact that everything has a natural desire to exist according to its own mode: thus the desire of a being capable of knowing follows upon the knowledge which it possesses; and since the human intellect apprehends being absolutely and according to all time, every intellectual being has a natural desire to exist always; a desire which cannot be void. Therefore the human soul is incorruptible. The angels and the souls of men are of different species, and naturally unequal; for subsistent forms do not multiply, and we must therefore admit that they differ in species. For separate forms are only one of a species; as is evident in the form of whiteness, which, if it were separate, would be only one. And diversities of species have always concomitant natural diversities of greater or less perfection or imperfection; for the differences which divide the same genus are contraries.