A nature is something's "what-ness", secondary substance, essence, sort, or kind, which can be held by different things (some separable and some inseparable). Without one's nature, one could not exist. One's nature is the most basic thing that can be said about something. If the nature allows for change or growth, then one's nature stays with itself throughout each of its stages of development. For example, an acorn and an oak tree are two different stages of development within the nature of "oak-ness". Humans, for example, may go from, say, a zygote, embryo, fetus, infant, child, teenager, adult while remaining human throughout. This shows that whatever is attained through an exercise of free agency is not one's nature. Jesus in Christian categories has two natures simultaneously - eternally divine and as of two thousand years ago, human.
- Nature (Catholic Encyclopedia)