Full Idea
'The nature of things' matters for the Stoics because they hold strong theses about the way things are: they are determinists, physicalists, teleologists.
Gist of Idea
Stoic 'nature' is deterministic, physical and teleological
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Julia Annas - The Morality of Happiness Ch.5
Book Reference
Annas,Julia: 'The Morality of Happiness' [OUP 1995], p.159