Full Idea
There's nothing better for the mind than morality, and a person ought to behave morally whether or not he owns Gyges' ring.
Gist of Idea
We should behave well even if invisible, for the health of the mind
Source
Plato (The Republic [c.374 BCE], 612b)
Book Reference
Plato: 'Republic', ed/tr. Waterfield,Robin [OUP 1993], p.368