Summa contra Gentiles2.94 Chapter XCIV. That the separate substance and the soul are not of one species

Furthermore it can be proved from the foregoing that the soul is not of the same species with separate substances.

For there is a greater difference between the human soul and a separate substance than between one separate substance and another. Now, separate substances are all specifically distinct from one another, as we have proved. Much more, therefore, is a separate substance specifically distinct from the soul.

Moreover. Each thing has its proper being according to its specific nature: because things which have a different kind of being have a different species. Now, the being of the human soul and that of a separate substance are not of the same kind; since the body cannot communicate in the being of a separate substance, whereas it can communicate in the being of the human soul, which is united in being to the body as form to matter. Therefore the human soul differs in species from separate substances.

Again. That which is specified by itself, cannot be of the same species as that which is not specified by itself but is part of a species. Now, the separate substance is specified by itself; whereas the soul is not, but is part of the human species. Hence it is impossible that the soul be of the same species as separate substances; except on the supposition that man be of the same species as they, which is clearly impossible.

Further. The species of a thing is gathered from its operation: since operation indicates the power which reveals the essence. Now, the proper operation of a separate substance and of the intellective soul is understanding. But the separate substance's mode of understanding is wholly different from that of the soul: because the soul understands by receiving from the phantasms; but not so the separate substance, since it has no corporeal organs, wherein phantasms must needs be. Therefore the human soul and the separate substance are not of the same species.