(d) If there are such things as composite substances, i.e. substances constituted by the substantial union of two really distinct principles, then it follows that while the composite substance itself is complete, each of its substantial constitutive principles is incomplete. Of course there are many philosophers nowadays who reject as mere mental fictions, as products of mere logical distinctions, and as devoid of objective validity, the notions of composite substance and incomplete substance. Nor is this to be wondered at when we remember what a variety of groundless and gratuitous notions are current in regard to substance itself ([64]). But understanding substance in the traditional sense already explained ([62]), there is nothing whatever inconsistent in the notion of a composite substance, or of an incomplete substance,—provided these notions are understood in the sense to be explained presently. Nay, more, not only are these notions intrinsically possible: we must even hold them to [pg 255] be objectively valid and real, to be truly expressive of the nature of reality, unless we are prepared to hold that there is no such thing as substantial change in the universe, and that man himself is a mere aggregate of material atoms moved according to mechanical laws and inhabited by a conscious soul, or thinking principle, rather than an individual being with one definite substantial nature.
What, then, are we to understand by complete and incomplete substances respectively? A substance is regarded as complete in the fullest sense when it is wanting in no substantial principle without which it would be incapable of existing and discharging all its functions in the actual order as an individual of some definite species. Of course no created substance exists or discharges its functions unless it is endowed with some accidents, e.g. with properties, faculties, forces, etc. But there is no question of these here. We are considering only the essential perfections of the substance. Thus, then, any existing individual of any species—a man, a horse, an oak—is a complete substance in this fullest sense. It is complete in the line of substance, in substantial perfection, “in ordine substantialitatis,” inasmuch as it can exist (and does actually exist) without being conjoined or united substantially with any other substance to form a composite substance other than itself. And it is complete in the line of specific perfection, “in ordine speciei,” because not only can it exist without such conjunction with any other substantial principle, but it can discharge all the functions natural to its species, and thus tend towards its final perfection ([47]) without such conjunction.
But it is conceivable that a substance might be complete in the line of substantial perfections, and thus be capable of existing in the actual order and discharging there some of the functions of its species without conjunction with any other substantial principle, and yet be incapable of discharging all the functions natural to an individual of its species without conjunction with some other substantial principle, in which case it would be incomplete in the line of specific perfection, though complete in everything pertaining to its substantiality. We know of one such substance,—the human soul. Being spiritual and immortal, it can exist apart from the body to which it is united by nature, and in this separated condition retain and exercise its spiritual faculties of intellect and will; it is therefore complete as regards the distinctively substantial perfection whereby it is “capable of [pg 256] existing in itself”. But being of its nature destined for union with a material principle, constituting an individual of the human species only by means of such union, and being capable of discharging some of the functions of this species, viz. the sentient and vegetative functions, only when so united, it has not all the perfections of its species independently of the body; and it is therefore an incomplete substance in the line of specific perfections, though complete in those essential to its substantiality.
Again, if it be true that just as man is composed of two substantial principles, soul and body, so every living thing is composed of a substantial vital principle and a substantial material principle, and that every inorganic individual thing is likewise composed of two really distinct substantial principles, a formative and a passive or material principle; and if, furthermore, it be true that apart from the spiritual principle in man every other vital or formative principle of the composite “things” of our experience is of such a nature that it cannot actually exist except in union with some material principle, and vice versa,—then it follows necessarily that all such substantial principles of these complete composite substances are themselves incomplete substances: and incomplete not only in regard to perfections which would make them subsisting individuals of a species, but (unlike the human soul) incomplete even in the line of substantiality itself, inasmuch as no one of them is capable of actually existing at all except in union with its connatural and correlative principle.
Thus we arrive at the notion of substances that are incomplete in the line of specific perfections, or in that of substantial perfections, or even in both lines. An incomplete substance, therefore, is not one which verifies the definition of substance only in part. The incomplete substance fully verifies the definition of a substance.[280] It is conjoined, no doubt, with another to form a complete substance; but it does not exist in the other, or in the composite substance, as accidents do. It is a substantial principle of the composite substance, not an accidental determination of the latter, or of the other substantial principle with which it is conjoined. It thus verifies the notion of substance as a mode of being which naturally exists in itself; and united with its correlative substantial principle it discharges the function of supporting all accidental determinations which affect the composite substantial [pg 257] essence. Since, however, it does not exist itself independently as an individual of a species, but only forms the complete individual substance by union with its correlative substantial principle, it may be, and has been, accurately described as not belonging to the category of substance formally, but only referentially, “reductivé”.
The concepts of composite substance, of complete and incomplete substances, understood as we have just explained them, are therefore perfectly intelligible in themselves. And this is all we are concerned to show in the present context. This is not the place to establish the theses of psychology and cosmology from which they are borrowed. That the human soul is spiritual and immortal; that its union with a really distinct material principle to form the individual human substance or nature is a substantial union; that all living organisms and all inorganic bodies are really composite substances and subject to substantial change: these various theses of scholastic philosophy we here assume to be true. And if they are true the conception of incomplete substances naturally united to form a complete composite substance is not only intelligible as an hypothesis but is objectively true and valid as a thesis; and thus the notion of an incomplete substance is not only a consistent and legitimate notion, but is also a notion which gives mental expression to an objective reality.
We may add this consideration: The concept of an accident really distinct from its substance involves no intrinsic repugnance. Yet an accident is a mode of being which is so weak and wanting in reality, if we may speak in such terms, that it cannot naturally exist except by inhering, mediately or immediately, in the stronger and more real mode of being which is substance. But an incomplete substance is a higher grade of reality than any accident. Therefore if accidents can be real, a fortiori incomplete substances can be real.
71. Substance and Nature.—We have already pointed out ([13]) that the terms “essence,” “substance,” and “nature” denote what is really the same thing, regarded under different aspects. The term “essence” is somewhat wider than “substance,” inasmuch as it means “what a thing is,” whether the thing be a substance, an accident, or a concrete existing individual including substance and accidents.
The traditional meaning of the term “nature” in Aristotelian [pg 258] and scholastic philosophy is unmistakable. It means the essence or substance of an individual person or thing, regarded as the fundamental principle of the latter's activities. Every finite individual comes into existence incomplete, having no doubt its essential perfections and properties actually, but its intermediate and final perfections only potentially ([47]). These it realizes gradually, through the exercise of its connatural activities. Every being is essentially intended for activity of some sort: “Omne ens est propter suam operationem,” says St. Thomas. And by the constant interplay of their activities these beings realize and sustain the universal order which makes the world a cosmos. There is in all things an immanent purpose or finality which enables us to speak of the whole system which they form as “Universal Nature”.[281]
Therefore what we call a substance or essence from the static point of view we call a nature when we consider it from the dynamic standpoint, or as an agent.[282] No doubt the forces, faculties and powers, the active and passive accidental principles, whereby such an agent exerts and undergoes action, are the proximate principles of all this action and change, but the remote and fundamental principle of the latter is the essence or substance of the agent itself, in other words its nature.