Hence the [20] soul must be a substance in the sense of the form of a natural body having life potentially within it. But substance2 is actuality, and thus soul is the actuality of a body as above characterized. Now the word actuality has two senses corresponding respectively to the possession of knowledge and the actual exercise of knowledge. It is obvious that the soul is actuality in the first sense, viz. that of knowledge as possessed, for both sleeping [25] and waking presuppose the existence