Summa contra Gentiles2.99 Chapter XCIX. That separate substances know material things

By these aforesaid forms, then, a separate substance knows not only other separate substances, but also the species of corporeal things.

For, since their intellect is perfect in respect of its natural perfection, being wholly in act, it follows that it comprehends its object, intelligible being to wit, in every respect. Now intelligible being comprises also the species of corporeal things. Therefore a separate substance knows them.

Again. Since the species of things are differentiated as the species of numbers, as stated above, it follows that the higher species contains in some way that which is in the lower, even as the greater number contains the lesser. Seeing then that separate substances are above corporeal substances, it follows that whatever is contained in corporeal substances materially, is contained in separate substances intelligibly; for that which is in a thing, is there according to the mode of the thing in which it is.

Again. If the separate substances move the heavenly bodies, as philosophers say, whatever results from the movement of the heavenly bodies is ascribed to those same bodies as instruments, since they move through being moved, and to the separate substances that move them, as principal agents. Now they act and move by their intellect. Consequently they cause whatever is done by the movement of the heavenly bodies, just as the craftsman works through his tools. Hence their forms which are generated and corrupted are in separate substances intelligibly. Wherefore Boethius, in his book De Trinitate, says that from forms that are without matter came the forms that are in matter. Therefore separate substances know not only separate substances, but also the species of material things. For if they know the species of bodies subject to generation and corruption as being the species of their proper effects, much more do they know the species of heavenly bodies, as being the species of their proper instruments.

Wherefore, since the intellect of a separate substance is in act, having all the images to which it is in potentiality; and since it has the power to comprehend all the species and differences of being, it follows of necessity that every separate substance knows all natural things and their whole order.

Yet seeing that the intellect in act is the thing actually understood, someone might think that a separate substance does not understand material things; for it would seem incongruous that a material thing should be the perfection of a separate substance.

But if the point be considered aright, the thing understood is a perfection of the one who understands, according to the image thereof in the intellect, for the stone which is outside the soul is not a perfection of our possible intellect. Now the image of the material thing is in the intellect of a separate substance immaterially, according to the mode of a separate substance, and not according to the mode of a material substance. Wherefore there is no reason why we should not say that this image is a perfection of the separate substance’s intellect, as its proper form.