When the brain is weary, when it begins to lose its elasticity and freshness, there will be the same lack of tonicity and strength in the brain product.

Some men often do a vast amount of literary work in entirely different lines during their spare hours.
Cessation of brain activity does not necessarily constitute brain rest, as most great thinkers know.

The men who accomplish the most brain-work, sooner or later--usually later, unfortunately--learn to give rest to one set of faculties and use another, as interest begins to flag and a sense of weariness comes.

In this way they have been enabled to astonish the world by their mental achievements, which is very largely a matter of skill in exercising alternate sets of faculties, allowing rest to some while giving healthy exercise to others.

The continual use of one set of faculties by an ambitious worker will soon bring him to grief.