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 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.