Full Idea
It is evident that the natural law is nothing else than the rational creature's participation of the eternal law.
Gist of Idea
Natural law is a rational creature's participation in eternal law
Source
Thomas Aquinas (Summa Theologicae [1265], I-II Q91 2)
Book Reference
Aquinas,Thomas: 'On Law, Morality and Politics', ed/tr. Baumgarth,W. /Regan R. [Hackett 1988], p.20
A Reaction
It is not enough merely that God decrees eternal laws. It is also necessary for us to use reason in order to participate. I'm not sure what reasoning process is involved.