Full Idea
None of the moral virtues is engendered in us by nature, since nothing that is what it is by nature can be made to behave differently by habituation. For instance, a stone, which has a natural tendency downwards, cannot be habituated to rise.
Gist of Idea
Moral virtue is not natural, because its behaviour can be changed, unlike a falling stone
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1103a19)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.91
A Reaction
Not much of an argument. Training a flower to grow up a drainpipe is not unnatural, but then the whole notion of 'unnatural' is hard to justify these days.