The true danger of every form of mental exercise is the addition of
worry. Laborious mental exercise is healthy unless it be made anxious by
necessary or unnecessary difficulties. Regular mental labour is best
carried on by introducing into it some variety. New work gives time for
repair better than attempt at complete rest, since the active mind finds
it impossible to evade its particular work unless its activity be
diverted into some new channel. During the new work a fresh portion of
the brain comes into play and the overwrought seat of mental faculty is
secured repose and recovery. Excessive competition in mental labour is
ruinous at all ages of life.