Full Idea
Protagoras and Hippias evidently believed that convention was an improvement on nature, whereas later sophists such as Antiphon, Thrasymachus and Callicles seemed to contend that conventional morality was undermined because it was 'against nature'.
Gist of Idea
Early sophists thought convention improved nature; later they said nature was diminished by it
Source
report of Protagoras (fragments/reports [c.441 BCE]) by Fred D. Miller jr - Classical Political Thought
Book Reference
'Encyclopedia of Classical Philosophy', ed/tr. Zeyl,Donald J. [Fitzroy Dearborn 1997], p.441
A Reaction
This gets to the heart of a much more interesting aspect of the nomos-physis (convention-nature) debate, rather than just a slanging match between relativists and the rest. The debate still goes on, over issues about the free market and intervention.