They are two categorically different things: the body is a three-dimensional object composed of the four elements, whereas the soul has no spatial dimensions. Soul is a kind of substance, participating in reason, fit for ruling the body. Augustine was not preoccupied, as Plato and Descartes were, with going too much into detail in his efforts to explain the metaphysics of the soul-body union. It sufficed for him to admit that they were metaphysically distinct. To be a human is to be a composite of soul and body, and that the soul is superior to the body. The latter statement is grounded in his hierarchical classification of things into those that merely exist, those that exist and live, and those that exist, live, and have intelligence or reason.