(_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 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.