No one who does not thoroughly know the modes of action which common experience has sanctioned is capable of judging of the circumstances which require a departure from those ordinary modes of action.
 John Stuart Mill, Considerations on Representative Government (1861). copy citation

Context

“It is true that a great statesman is he who knows when to depart from traditions, as well as when to adhere to them; but it is a great mistake to suppose that he will do this better for being ignorant of the traditions. No one who does not thoroughly know the modes of action which common experience has sanctioned is capable of judging of the circumstances which require a departure from those ordinary modes of action. The interests dependent on the acts done by a public department, the consequences liable to follow from any particular mode of conducting it, require for weighing and estimating them a kind of knowledge, and of specially exercised judgment, almost as rarely found in those not bred to it, as the capacity to reform the law in those who have not professionally studied it.” source