Summa contra Gentiles2.71 Chapter LXXI. That the soul is united to the body immediately

We are able to conclude from the foregoing that the soul is united to the body immediately, nor must we admit any medium as uniting the soul to the body; whether it be the phantasms, as Averroes maintained; or its powers, as some say; or the corporeal spirit, as others have asserted.

For it has been proved that the soul is united to the body as its form. Now a form is united to matter without any medium whatever: since to be the act of such and such a body is competent to a form by its very nature and not by anything else. Consequently neither is there anything that makes one thing out of matter and form, except the agent which reduces the potentiality to act, as Aristotle proves (8 Metaph.) for matter and form are related as potentiality and act.

It may be said however that there is a medium between the soul and the body, not in the point of being, but as regards movement and in the order of generation. As regards movement, since in the movement whereby the soul moves the body there is a certain order among moved and movers. For the soul produces all its operations through its powers, so that it moves the body by means of its power, and again the members by means of the vital spirit, and again one organ by means of another. In the order of generation dispositions to a form precede the form in matter, although they are posterior thereto in being. Consequently the body’s dispositions whereby it is rendered the proper perfectible subject of such and such a form, may in that sense be described as a medium between the soul and body.