To be under the eyes of others—to have to defend oneself to others—is never more important than to those who act in opposition to the opinion of others, for it obliges them to have sure ground of their own.
 John Stuart Mill, Considerations on Representative Government (1861). copy citation

Context

“The notion is itself unfounded that publicity, and the sense of being answerable to the public, are of no use unless the public are qualified to form a sound judgment. It is a very superficial view of the utility of public opinion to suppose that it does good only when it succeeds in enforcing a servile conformity to itself. To be under the eyes of others—to have to defend oneself to others—is never more important than to those who act in opposition to the opinion of others, for it obliges them to have sure ground of their own. Nothing has so steadying an influence as working against pressure. Unless when under the temporary sway of passionate excitement, no one will do that which he expects to be greatly blamed for, unless from a preconceived and fixed purpose of his own, which is always evidence of a thoughtful and deliberate character, and, except in radically bad men, generally proceeds from sincere and strong personal convictions.” source