The proper work of the mind is the exercise of choice

From The Daily Stoic by Ryan Holiday and Stephen Hanselman. 

The proper work of the mind is the exercise of choice, refusal, yearning, repulsion, preparation, purpose, and assent. What then can pollute and clog the mind’s proper functioning? Nothing but its own corrupt decisions.

—EPICTETUS, DISCOURSES, 4.11.6–7