Full Idea
A man can best conduct himself in harmony with justice, if when in company of witnesses he upholds the laws, and when alone without witnesses he upholds the edicts of nature.
Clarification
'Law' is the Greek word 'nomos', which also translates as 'convention'
Gist of Idea
We should follow the law in public, and nature in private
Source
Antiphon (fragments/reports [c.439 BCE], B44), quoted by Anon (Oxy) - Oxyrhynchus Papyrus XI.1364
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.147
A Reaction
I'm not sure how you identify the 'edicts of nature', without guidance from other people or the law. Natural behaviour can be pretty grim.