An Aristotelian proof of the quidditative definition of the soul

(Based on Aquinas, In De Anima, lb. 2, lc. 2.)

1. The soul is the principle of life, i.e., that on account of which whatever lives is alive. (Nominal definition of ‘soul’, specifying what we should mean by the name, even though we may not know what sort of thing it is to which this name according to this signification applies.)

2. But for a living body to live is for it to exist simpliciter (i.e., without qualification). (Clear from the fact that birth and death are for living things to begin and to cease to be.)

3. Therefore, the soul is that on account of which a living being exists simpliciter. (Conclusion of 1 and 2.)

4. But that on account which something exists simpliciter is its substantial form. (Nominal definition of ‘substantial form’.)

5. Therefore, the soul is the substantial form of a living body. (Conclusion of 3 and 4. This is the quidditative definition of the soul, which says what sort of thing it is to which the name ‘soul’ applies in the signification expressed by its nominal definition in 1.)