An error about potency

St. Thomas raises an objection to saying that the man has a single existence through his soul. I am most interested with what St. Thomas is saying about the relation of the soul (an act) to the body (a potency).

Aristotle says in the second book of the De Anima that the soul is the act of a of a physical body with organs. That which stands to the soul as matter to act, therefore, is already a physical body with organs: which cannot be except through a form by which it is constituted in the genus of body.

philosophus dicit in II de anima, quod anima est actus corporis physici organici. Hoc igitur quod comparatur ad animam ut materia ad actum, est iam corpus physicum organicum: quod non potest esse nisi per aliquam formam, qua constituatur in genere corporis.

He responds saying

Sometimes in the definitions of forms the subject is posited as unformed; as when it is said motion is act existing in potency. Sometimes the subject is posited as having been formed, as when it is said motion is the act of a mobile, light is the act of the shining. In this way the soul is an act of a physical body with organs, because the soul makes it to be a body with organs, just as light makes something shining.

in definitionibus formarum aliquando ponitur subiectum ut informe, sicut cum dicitur: motus est actus existentis in potentia. Aliquando autem ponitur subiectum formatum, sicut cum dicitur: motus est actus mobilis, lumen est actus lucidi. Et hoc modo dicitur anima actus corporis organici physici, quia anima facit ipsum esse corpus organicum, sicut lumen facit aliquid esse lucidum.

This is an important distinction with a wider application. The distinction between act and potency is easily perverted when we see potency in the second way that St. Thomas speaks of, sc. as already informed. Seen in this way, we easily relate to potency as having a form that it lacks in itself-  and in so doing we miss what is crucial to it as potency. This leaves us with another worthless dualism that contemporary thinkers are perpetually bemoaning (although the real problem with dualism was said best by Democritus “you can’t make one out of two or two out of one”- which is exactly what the real distinction between potency and act denies)

One wishes the principle were invoked more in discussions of esse and essentia. It is also relevant to the discussion of causality (cause and effect are act and potency)

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