The types of composition in creatures distinguished by Aquinas
After distinguishing the two senses of ‘being’ (for an explanation of this distinction check my lecture-note ‘Aquinas on Being and Essence’), and pointing out that it is only beings in the primary sense that have essence, since essence is precisely the determination of an act of being signified by the term ‘being’ in this sense, Aquinas goes on to show that the essence of a material substance itself has to contain matter somehow. The reason why there has to be matter in their essence is that the essence of a thing is what is signified by the thing’s quidditative definition, the definition consisting of the thing’s genus and specific difference, which, in turn are definable further in terms of the more and more universal differences and genera, climbing up the Porphyrian tree. But in the last analysis any such definition of any material substance has to contain the difference ‘material’. Therefore, at least this part of all these definitions signifies matter somehow, and thus, the definition itself, which signifies the whole essence of the thing, signifies matter as a part of this essence. (The other part is substantial form, conceived as that which informs this matter, giving it its substantial being.) But since the definition signifies matter in this way only in a universal manner, and never the particular, designated matter of this or that individual as such, the essence of a material substance contains matter only in this universal manner. Such an essence, therefore, contains matter only in general, but of course this does not mean that it should contain some “general matter”. There is no such a thing as “general matter”, there is only particular matter; but this can be conceived in a universal manner, and it is matter thus conceived that is said to be a part of a material essence.
On the other hand, what really exists in things is only particular, designated matter, that is, matter here and now, considered under the particular dimensions of the extension of this particular body. It is this matter, considered as such, which Aquinas identifies as the principle of individuation in material substances. This matter is obviously merely conceptually distinct from the matter considered in general, which is conceived as a part of the essence of this thing. But this particular matter is really distinct from the essence of the same thing itself, for it is precisely this particular matter that is excluded from the concept of essence, as it is signified by the abstract form of an essential predicate of the thing. (That is to say, as it is signified by the term ‘humanity’, the abstract form of the essential predicate ‘man’. Concerning the difference between a real distinction as opposed to a merely conceptual distinction or distinction of reason, see the handout accompanying the lecture note on Aquinas.) On the other hand, Aquinas also argues that the different essential predicates of the same particular thing do not signify really distinct forms in the same thing. This is Aquinas’ famous doctrine of the unity of substantial forms: in one and the same thing it is one and the same substantial form that verifies of this thing all the essential predicates of the thing. (Thus, here we also have a merely conceptual distinction between the forms signified by the different substantial predicates of the same thing: the predicates arranged on the Porphyrian tree signify one and the same form in the same thing, but they signify this form according to different, more or less abstract, and consequently more or less universal concepts. Note here that Aquinas sometimes also refers to a concept by the term ‘intention’. In this context ‘concept’ and ‘intention’ mean the same: (the direct, immediate object of) an act of mind on account of which the mind has some abstract, universal cognition of several particulars.)
On the basis of these considerations, we have to say that material substances are composite in several ways, depending on how we consider and identify their various integral parts.
1. They are composed of matter and (substantial) form, as is obvious
2. Their essence itself is also composed of matter and form considered in general
3. They are also composed of their essence (which comprises their matter and form in general) and their individual, designated matter, which is the principle of their individuation, i.e., that on account of which one individual of the same species is numerically distinct from another individual of the same species.
These three types of composition are peculiar to material substances, since all of these are the result of their having matter, informed by their substantial form.
But they also exhibit two further sorts of composition, which they share even with immaterial substances, except for God. These are
4. The composition from subject and accident
5. The composition of essence and existence (potentiality and actuality)
The former type of composition is present even in angels (“intelligences”, as Aquinas also refers to them), given the fact that even angels are changeable in respect of their spiritual activity, say, changing the objects of their thought or their will. It is only God, who is eternally immutable, self-thinking thought, who knows of all changeable things by understanding His own nature, which is only fragmentarily and imperfectly represented by the finite natures of His creatures, just as the light of the sun can be reflected by several, brighter and dimmer, variously tinted mirrors.
The second type of composition also has to be present in all creatures given that their essence is really distinct from their existence (this fundamental Thomistic thesis is often referred to as “the thesis of real distinction” [between essence and existence in creatures]). It is only God whose essence is nothing but His existence, which is precisely the reason why His essence, not being distinct from His existence, does not put any limitation on the infinite actuality of His existence. By contrast, the essences of creatures, even of the highest-ranking angels, are some determination of the act of existence which actualizes this essence. Indeed, given that angels cannot be numerically different on account of their designated matter (since they are immaterial), they differ from one another in their essence, that is, in virtue of the differences between how much limitation their essence imposes upon their existence: thus they differ in their essential perfection, and so they have to differ not only numerically but also specifically; according to Thomas, therefore, there cannot be two angels of the same species.
It is possible, however, to have several human souls of the same species, despite the fact that according to Aquinas, human souls are also immaterial substances. However, they are immaterial not in exactly the same sense in which separate substances are immaterial. Separate substances (angels and God) are immaterial in the sense that their being is in no way the actuality of any parcel of matter. The human soul, however, acquires its being in some parcel of matter, yet it is immaterial in that it is not dependent for its continued existence upon actually informing any matter. This is why it is possible for human souls to multiply in the same species according the multiplicity of human bodies they inform, and to continue in their individual existence even after they get separated from the human body upon the death of a human person. (Check Aquinas quoting Avicenna for this solution.)
It is this different interpretation of the immateriality of the human soul that allows Aquinas to avoid the otherwise apparently inevitable Averroist conclusion according to which there is only one human intellect, a separate substance, which is united with humans only in its operation, by using individual humans’ phantasms for its thinking. But by insisting that the human intellect has to be united with the body not only in its operation, but also in its being, Aquinas can maintain the numerical multiplication of human intellects, and thus he can avoid the further unorthodox implications of the Averroist position.