Frank J. Wilstach, comp. A Dictionary of Similes. 1916.
Discipline
Discipline, like the bridle in the hand of a good rider, should exercise its influence without appearing to do so, should be ever active, both as a support and as a restraint, yet seem to lie easily in hand. It must always be ready to check or pull up, as occasion may require; and only when the horse is a runaway should the action of the curb be perceptible.
—Julius. C. Hare