Full Idea
System T is a normal modal system augmented with the reflexivity-generating axiom □p→p, and is, I think, the best modal logic for modeling lawhood.
Gist of Idea
T adds □p→p for reflexivity, and is ideal for modeling lawhood
Source
Jonathan Schaffer (Causation and Laws of Nature [2008], n46)
Book Reference
'Contemporary Debates in Metaphysics', ed/tr. Sider/Hawthorne/Zimmerman [Blackwell 2008], p.104
A Reaction
Schaffer shows in the article why transitivity would not be appropriate for lawhood.