Full Idea
True opinions are a fine thing and all they do is good, …but they escape from a man's mind, so they are not worth much until one ties them down by (giving) an account of the reason why.
Gist of Idea
True opinions only become really valuable when they are tied down by reasons
Source
Plato (Meno [c.376 BCE], 98a3)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.895
A Reaction
This gives justification the role of guarantee, stabilising and securing true beliefs (rather than triggering some new thing called 'knowledge').