5. The act of understanding cannot be the act of anything corporeal. But it is an act of the soul. Therefore the intellectual soul at least is not a body.
It is easy to solve the arguments whereby some have endeavoured to prove that the soul is a body. They point such facts as these, -- that the son resembles the father even in the accidents of his soul, being generated from the father by severance of bodily substance; and that the soul suffers with the body; and is separated from the body, separation supposing previous bodily contact. Against these instances we observe that bodily temperament is a sort of predisposing cause of affections of the soul: that the soul suffers with the body only accidentally, as being the form of the body: also that the soul is separated from the body, not as touching from touched, but as form from matter; although there is a certain contact possible between an incorporeal being and the body, as has been shown above (Chap. LVI).
Many have been moved to this position by their belief that what is not a material body has no existence, being unable to transcend the imagination, which deals only with material bodies. Hence this opinion is proposed in the person of the unwise: The breath of our nostrils is smoke, and reason a spark in the beating of the heart (Wisdom ii, 2).
2.64 : That the Soul is not a Harmony
2.66 : Against those who suppose Intellect and Sense to be the same