Summa contra Gentiles2.63 Chapter LXIII. That the soul is not a temperament, as Galen asserted

The opinion of the physician Galen about the soul is akin to the aforesaid opinion of Alexander concerning the possible intellect.

For he says that the soul is a temperaments He was moved to make this assertion by the fact that we see resulting from various temperaments in us, various passions which are ascribed to the soul: for some who have, for example, a choleric temperament, are easily angered, while melancholic persons are prone to be sad. Consequently the same arguments avail to disprove this opinion, as were adduced against the opinion of Alexander, as well as some that apply specially thereto.

For it was proved above that the operation of the vegetative soul, sensitive knowledge and, much more, the operation of the intellect surpass the power of the active and passive qualities. Therefore the temperament cannot be the principle of the soul’s operations: and consequently it is impossible for the soul to be the temperament.

Again. Seeing that the temperament is something set up by contrary qualities as a kind of mean between them, it cannot possibly be a substantial form; because substance has no contrary, nor is it a recipient of more or lessS But the soul is a substantial, not an accidental, form: else a thing would not obtain species or form from its soul. Therefore the soul is not the temperament.

Further. The temperament does not move an animal’s body by local movement: for it would follow the movement of the predominant element, and thus would always be moved downwards. But the soul moves the body in all directions. Therefore the soul is not the temperament.

Moreover. The soul rules the body, and curbs the passions that result from the temperament. For by temperament some are more prone than others to desire or anger, and yet refrain more from these things, on account of something that keeps them in check, as may be seen in those who are continent. But the temperament does not this. Therefore the soul is not the temperament.

Apparently he was deceived through failing to observe that passions are ascribed to the temperament in one way, and to the soul in another. For they are ascribed to the temperament as causing a disposition, and in respect of that which is material in the passions, for instance the heat of the blood and the like; whereas they are ascribed to the soul as their principal cause, and in respect of what is formal in the passions, for instance the desire of vengeance in anger.