Sympatheia is the Stoic conviction that all things are bound together into a single whole, and that human beings in particular share one nature and belong to one community. The Greek word means, roughly, a fellow-feeling or interconnection running through the entire cosmos, which the Stoics understood as a rational, living order in which every part is related to every other. From this cosmic picture they drew a very practical ethics: because we are all parts of one rational nature, we are made for cooperation, and to work against one another is to work against the grain of the world itself.
Marcus Aurelius returns to this thought constantly. He tells himself that human beings exist for the sake of one another, and compares people who obstruct and resent each other to the parts of a single body turning against themselves. We are made for cooperation, he writes, like feet, like hands, like eyelids, like the rows of upper and lower teeth; to act against each other is contrary to nature. Elsewhere he pictures all rational beings as citizens of one great city, so that when he calls himself a citizen of Rome he means it only in a small way — in the largest sense, his city is the world, and every other person is a fellow citizen of it.
The practical force of sympatheia is that it dissolves the sense of separateness that fuels so much friction. The stranger, the difficult colleague, the person on the other side of an argument — none of them is finally an alien or an obstacle. Each is a limb of the same body you belong to, working, like you, out of their own understanding, however flawed. Marcus even instructs himself, when someone wrongs him, to remember their kinship: that they share the same reason and the same origin, that they were not born to harm him, and that anger at them is like the hand raging at the foot.
Held as a daily practice, sympatheia reshapes how you meet other people. Before writing someone off, you remember that you are made for cooperation with them, not competition against them; that their good and yours are not finally at odds; and that a small kindness or patience is not a loss but the natural function of a part that knows it belongs to a whole. It is a quiet corrective to the reflex of every-man-for-himself — a reminder that we were made to work together, and are least ourselves when we forget it.