Summa contra Gentiles2.51 Chapter LI. That the intellectual substance is not a material form

From the same premisses it may be shown that intellectual natures are subsistent forms, and do not exist in matter as though their being depended on matter.

Because forms dependent on matter as regards their being properly speaking have not being themselves, but the composites through them. Hence if intellectual substances were forms of this kind, it would follow that they have material being, just as they would if they were composed of matter and form.

Again. Forms that subsist not of themselves cannot act of themselves, but the composites act through them. If therefore intellectual natures were forms of this kind, it would follow that they do not themselves understand, but the things composed of them and matter. Consequently an intelligent being would be composed of matter and form. And this has been proved to be impossible.

Moreover. If the intellect were a form in matter and not self-subsistent, it would follow that what is received into the intellect is received into matter: because such forms as have their being tied to matter, do not receive anything without its being received into matter. Since, then, the reception of forms into the intellect is not a reception of forms into matter, it is impossible that the intellect be a material form.

Further. To say that the intellect is a non-subsistent form and buried in matter, is the same in reality as to say that the intellect is composed of matter and form, and the difference is merely nominal: for in the former case the intellect will be indicated as the form of the composite, while in the latter, the intellect denotes the composite itself. Wherefore if it is false that the intellect be composed of matter and form, it will be false that it is a non-subsistent and material form.