Why does Marcus Aurelius say human beings are made for cooperation?

Marcus opens Meditations 2.1 by rehearsing the kind of person he expects to meet before he has even left his bed: the busy-body, the ungrateful, the arrogant, the deceitful, the envious, the unsocial. This is not cynicism. It is preparation. He wants to meet these people without surprise and without anger, and his reason is that anger toward them misunderstands what a human being is.

His argument runs through three claims stacked on top of each other. First, wrongdoing comes from ignorance: people who act badly do so "by reason of their ignorance of what is good and evil" (Meditations 2.1). They have mistaken values, not a defective will aimed at hurting him personally. Second, the wrongdoer is not a stranger to him but a kinsman, sharing not just blood but "the same intelligence and the same portion of the divinity" (Meditations 2.1). For Marcus, all rational beings participate in the same universal reason, so an offender is not an alien threat but a relative who has gone astray. Third, and this is the line the question is about, Marcus says "we are made for co-operation, like feet, like hands, like eyelids, like the rows of the upper and lower teeth" (Meditations 2.1).

The comparison is doing real work. Feet, hands, eyelids and the two rows of teeth are not cooperative by choice or by good mood. They are cooperative by design. A hand that closed against the other hand, or an eyelid that fought the eye it was meant to protect, would not be expressing some rare defect of character. It would be failing at the very thing it is for. Marcus is saying that human cooperation has the same status. It is not a virtue added on top of human nature. It is what human nature already is, the way grasping is what a hand already is.

This corrects a common misreading of Stoic ethics as a philosophy of self-sufficient withdrawal, the sage alone with his reason, indifferent to everyone else. Marcus draws the opposite conclusion from the same premises. Because externals like reputation, pain, and dishonor are neither good nor evil, and fall on good and bad people alike (Meditations 2.11), nothing the busy-body or the deceitful person does can actually damage him. That freedom from harm is not a reason to disengage from other people. It is what allows him to stay engaged without becoming corrupted by them. He writes that he can neither be injured by the wrongdoer nor be angry with him, precisely because he has seen what the wrongdoer's nature really is and what his own nature demands of him (Meditations 2.1).

The conclusion follows directly: "To act against one another then is contrary to nature; and it is acting against one another to be vexed and to turn away" (Meditations 2.1). Notice the two things Marcus counts as acting against nature. One is obvious: attacking someone, undermining them, working against their interests. The other is easy to miss: simply turning away, being vexed and withdrawing. Irritated avoidance is, on his account, its own kind of failure to cooperate, not a neutral or safe alternative to conflict.

What changes if you take this seriously is not your tolerance for bad behavior but your diagnosis of it. The difficult colleague or ungrateful relative has not damaged your good, because your good was never in their hands. What is up to you is whether you meet them as a hand meets a hand, still working the same task, or as one tooth trying to bite the row across from it. The next time you feel the pull to turn away from someone, ask what part of the same body you and this person are still supposed to be.