Full Idea
Protagoras claimed that virtue was teachable, but now tries to show it is not knowledge, which makes it less likely to be teachable.
Gist of Idea
Protagoras contradicts himself by saying virtue is teachable, but then that it is not knowledge
Source
comment on Protagoras (fragments/reports [c.441 BCE]) by Plato - Protagoras 361b
Book Reference
Plato: 'Protagoras and Meno', ed/tr. Guthrie,W K C [Penguin 1956], p.99