The Stoics gave us one of history's most practical psychological frameworks: divide the world into what you can control and what you cannot. Your judgments, choices, and responses belong to you. Everything else—weather, illness, other people's actions—falls outside your power. Direct your efforts inward, toward the only domain where effort has purchase.

This is wisdom. It has saved countless people from fruitless anxiety about things beyond their reach. But it contains a hidden flaw—one that becomes painfully apparent when we try to apply it to our most important relationships.

In the Greek philosophical tradition, the fundamental distinction was between tyche (chance, fortune, what happens to us) and psyche (soul, the seat of deliberation and choice). The Stoics codified this into their famous dichotomy of control.

For them, the binary was absolute. Epictetus opens his Enchiridion: "Some things are within our power, while others are not." Within our power: our opinions, impulses, desires. Not within our power: our body, property, reputation—whatever is not our own doing.

Notice what disappears in this framework: the distinction between a lightning strike and a betrayal. Between a tumor and a lie. Between........

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