Summa contra Gentiles2.95 Chapter XCV. How we are to understand genus and species in separate substances

It is necessary to consider wherein species differ in separate substances. For in material things of the same genus and differing in species, the ratio of the genus is derived from the material principle, and the specific difference from the formal principle. Thus the sensitive nature, whence is derived the ratio of animal, is, in man, material in respect of the intellective nature, whence is derived the specific difference of man, namely rational. Consequently, if separate substances are not composed of matter and form, as is evident from what has been said, it is not clear how we are to ascribe to them genus and specific difference.

Accordingly it must be observed that the various species of things possess the nature of being in degrees. For in the first division of being we find at the very outset something perfect, namely substantial (per se) being and actual being, and something imperfect, namely, accidental being and potential being. In like manner if we run through the various species, we find that one species has an additional grade of perfection over another, for instance animals over plants, and animals endowed with locomotion over those that are immovable. Again, in colours one species is seen to be more perfect than another, according as it approaches to whiteness. Hence Aristotle (8 Metaph.y says that the definitions of things are like a number, the species of which is changed by the subtraction or addition of unity: in the same way as a different species results in definitions, if a difference be removed or added. Wherefore the ratio of a determinate species consists in this, that the common nature is placed in a determinate degree of being. And since in things composed of matter and form, the form is the term as it were, and that which is determined thereby is the matter or something material: it follows that the ratio of the genus must be taken from the material, and the specific difference from the formal element. Hence there results one thing from difference and genus, even as from matter, and form. And just as it is one and the same nature that results from matter and form, so the difference does not add an extraneous nature to the genus, but is a determination of the generic nature itself: for instance if we take as a genus an animal with feet, a difference thereof will be an animal with two feet, which difference clearly adds nothing extraneous to the genus.

It is therefore evident that it is accidental to the genus and difference, that the determination denoted by the difference be caused by a principle other than the generic nature, since the nature signified by the definition is composed of matter as determined, and form as determining. Hence if there be a simple nature, it will be determined by itself, nor will it need to have two parts, one determining and the other determined. Consequently the ratio of genus will be derived from the ratio of its nature, and its specific difference will be derived from its determination in that it is placed in a determinate grade of being.

It also follows from this that if any nature be without limits and infinite in itself, as we have shown to be the case with the divine nature, we cannot ascribe to it either genus or species: and this agrees with what we have proved about God.

Moreover, since the difference of species is attributed to separate substances according as various degrees are ascribed to them, and since there are not several individuals in one species, it is clear, from what has been said, that no two separate substances are equal in degree, but that one is naturally above another. Hence it is stated (Job xxxviii. 33): Dost thou know the order of heaven? And Dionysius says (Ccel. Hier. x.) that even as in the whole multitude of angels there is a supreme, middle, and lowest hierarchy, so in each hierarchy there is a highest, a middle, and a lowest order, and in each order, highest, middle, and lowest angels.

Hereby is excluded the opinion of Origen, who said that all spiritual substances were created equal from the beginning, among which he reckoned even souls; and that the difference which we find among these substances, in that this one is united to a body, and that one not, that this one is higher and that one lower, results from a difference of merits. For we have shown that this difference of degree is natural; that the soul is not of the same species as separate substances; nor the separate substances themselves of the same species with one another, and that they are not equal in the order of nature.