Summa contra Gentiles2.93 Chapter XCIII. That there are not several separate substances of one species

From what has been said concerning these substances, it may be shown that there are not several separate substances of one species.

For it has been proved above that separate substances are subsistent quiddities. Now, the species of a thing is signified by its definition, for this is the sign of a thing's quiddity. Hence a subsistent quiddity is a subsistent species. Therefore there cannot be several separate substances unless they be several species.

Further. Whatever things are the same in species but differ numerically, have matter: since a difference resulting from form, involves a specific difference: whereas that which results from matter, causes a diversity of number. Now, separate substances are utterly devoid of matter, whether as part of themselves, or by being united to matter as its form.- Therefore they cannot possibly be several of one species.

Moreover. The purpose for which, in corruptible things, there are several individuals in one species, is that the specific nature which cannot be preserved for ever in one individual, may be preserved in many: wherefore even in incorruptible bodies there is but one individual in one species. Now the nature of a separate substance can be preserved in one individual, since they are incorruptible, as we proved above. Therefore there is no need for several individuals of the same species in those substances.

Again. In each individual, that which belongs to the species is more noble than that which is the principle of individuality, existing apart from the specific nature. Consequently the multiplicity of species adds more nobility to the universe, than the multiplicity of individuals of one species. Now, the perfection of the universe applies especially to separate substances. Therefore it is more in keeping with the perfection of the universe, that they should be many differing in species, than that they should be multiplied numerically within the same species.

Further. Separate substances are more perfect than the heavenly bodies. Now, in the heavenly bodies, by reason of their perfection, one species contains but one individual: both because each one of them consists of the whole matter pertaining to its species, and because the one individual possesses perfectly the power of its species for the fulfilment of the purpose to which that species is directed in the universe, as especially may be seen in the sun and moon. Much more therefore should we find but one individual of the one species in the separate substances.