Now, whereas we have already elsewhere distinguished what are called the parts of the soul, and whereas the nutritive is, in all living bodies, capable of existing without the other parts, while none of the others can exist without the nutritive; it is clear that sleep and waking are not affections of such living things as partake only of growth and decay, eg not of plants, because these have not the faculty of sense perception, whether or not this be capable of separate existence; in potentiality, indeed, and in being it is separable.