The core of stoicism
From Marcus Aurelius - Meditations:
Two points in the Stoic system deserve special mention.
One is a careful distinction between things which are in our power and things which are not. Desire and dislike, opinion and affection, are within the power of the will; whereas health, wealth, honour, and other such are generally not so.
The Stoic was called upon to control his desires and affections, and to guide his opinion; to bring his whole being under the sway of the will or leading principle, just as the universe is guided and governed by divine Providence.