Full Idea
The guardians of the state should aim to get an exact idea of the common element in all the four virtues.
Clarification
'Virtue' here is the Greek word 'areté', which also translates as 'excellence'
Gist of Idea
The Guardians must aim to discover the common element in the four cardinal virtues
Source
Plato (The Laws [c.349 BCE], 965d)
Book Reference
Plato: 'The Laws', ed/tr. Saunders,Trevor J [Penguin 1970], p.525