In the case of a human, Aristotle spoke of the soul as the “first” actuality of the body, and of intellect as a second actuality of a human being. This was extrapolated by later commentators into a broader concept of second nature. Nature for Aristotle is not just the way something statically is, or a set of abstract laws; it is an internal source of motion and rest within each natural thing. In the case of an animal, it is responsible for growth and characteristic bodily movements or behavior. I have glossed actuality (energeia) as at-work-ness, or a status of being effectively operative in a process, so there is a kind of metonymic relation between nature and actuality.
The idea that nature or actuality is something admitting of structural degrees seems very useful. Modern discourse is full of awkward contrasts between, e.g., nature and culture, as if these were mutually exclusive domains. But culture or character or mind exists within — or layered on top of — what we would call physical nature. It is a relatively autonomous additional layer with additional capabilities, that would not exist without the first layer. It is a complex adverbial modification of the active processes associated with first nature. Hegelian Spirit is a thing of this kind. (See also Ethos, Hexis; Rational/Talking Animal; Parts of the Soul; Alienation, Second Nature.)
The Postmodern Peripatetic 