Full Idea
Our present argument is about …the Equal, the Beautiful itself, the Good itself, the Just, the Pious, and about all those things to which we can attach the word 'itself', both when we are putting questions and when we are answering them.
Gist of Idea
The Forms arise whenever we talk of something 'in itself'.
Source
Plato (Phaedo [c.374 BCE], 75d)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.66
A Reaction
This identifies the Forms with ideas which emerge during philosophical conversation (either the 'elenchus' interrogation or the 'dialectic' discussion). So they arise from using language. The 'itself' test works quite well in English. Cf essentialism.