The most recognizable Stoic teaching is the dichotomy of control: some things are up to us (judgments, choices, intentions, desires, aversions) and others are not (the body, reputation, others' actions, the weather, the past). Energy spent on the latter is wasted; energy spent on the former is the only true freedom. The modern philosopher William Irvine refined this into a trichotomy: some things are entirely within our control, some entirely outside it, and some only partially. Internal goal setting, aiming at the quality of one's effort rather than at outcomes, pairs effort with what one can actually deliver, so that a tennis player aims to play the best match possible rather than to win.
This dichotomy is operationalized through three disciplines, sometimes called the three Stoic topoi. The discipline of assent trains the practitioner to withhold agreement from impressions, called phantasiai, until they have been examined. Stoics classified impressions as cataleptic (clear, true, and grasping reality) or non-cataleptic and uncertain; only the first kind warrants assent without further thought. The standard practice is to greet each impression and test it: "You appear to me, impression; let me examine you. Are you what you seem?" A related exercise is objective representation, stripping a feared event of dramatic coloring: "a man is dying" rather than "the unspeakable tragedy of death." Epictetus summed up the underlying conviction in a single sentence: "It is not the things themselves that disturb us, but our opinions about the things."
The discipline of action calls for acting with reasoned intention, in accordance with virtue and the common good, while reserving assent to outcomes through the reserve clause: "I will do this, fate permitting." Effort is yours; results are not. The discipline of desire aligns wanting and aversion with what is actually within your power: want only what you control, and accept what happens as part of the rational order. This is the source of amor fati, love of fate, embracing whatever happens as necessary rather than wishing things were otherwise. Epictetus put it memorably: "Don't ask things to happen as you wish; wish them to happen as they do." Marcus Aurelius restated the central conviction plainly: you have power over your mind, not outside events; realizing this is the beginning of freedom.