Full Idea
True opinion is as good a guide as knowledge for the purpose of acting rightly.
Clarification
'Knowledge' here is the Greek word 'epistemé'
Gist of Idea
As a guide to action, true opinion is as good as knowledge
Source
Plato (Meno [c.376 BCE], 97b)
Book Reference
Plato: 'Protagoras and Meno', ed/tr. Guthrie,W K C [Penguin 1956], p.153
A Reaction
This is the germ of Peirce's epistemology - that knowledge is an interesting theoretical concept, but opinion/belief is what matters, and most needs explanation.