Full Idea
Plato's theory of Forms allowed him to claim that the sophists and other opponents were trapped in the world of appearance. What they therefore taught was only apparent wisdom and virtue.
Gist of Idea
Plato's Forms meant that the sophists only taught the appearance of wisdom and virtue
Source
report of Plato (works [c.375 BCE]) by Alexander Nehamas - Eristic,Antilogic,Sophistic,Dialectic p.118
Book Reference
Nehamas,Alexander: 'Virtues of Authenticity' [Princeton 1999], p.118