Full Idea
The Cynics were convinced of the purely conventional foundation of Athenian values, which meant they had no rational foundation at all. They therefore rejected them in favour of what is correct and worthwhile by nature.
Gist of Idea
The Cynics rejected what is conventional as irrational, and aimed to live by nature
Source
comment on Diogenes (Sin) (reports [c.360 BCE]) by Richard Taylor - Virtue Ethics: an Introduction Ch.8
Book Reference
Taylor,Richard: 'Virtue Ethics: an Introduction' [Prometheus 2002], p.47
A Reaction
This shows how the Cynics are key players in the progress of the nomos-physis debate, which keeps resurfacing as relativism vs absolutism, cognitivism vs non-cognitivism, and even romanticism vs classicism. The trouble is, convention is natural!