In this paper, I propose a solution to a challenge formulated by Judith Jarvis Thomson: We have to explain why the moral asymmetry between doing and allowing harm is a deep feature of our moral thinking. In a nutshell, my solution is this: It could not be otherwise. Accepting the asymmetry is indispensable for the construction and maintenance of stable moral communities. -/- My argument centrally involves mental resource management. Moral communities depend on their members’ commitment to moral norms. And, (...) I argue, community members can only be committed to these norms in the required ways without overextending their mental resources if morality contains the doing/allowing asymmetry. -/- This has two noteworthy implications. Firstly, we can and should stop discussing the asymmetry in normative ethics and, instead, regard it as “moral axiom” we cannot but accept. Secondly, even consequentialists have to accept the asymmetry but, by doing so, their view loses part of its intuitive appeal. Lastly, I turn to an objection. One might think that my indispensability argument confuses issues in moral psychology with issues in normative ethics. I reject the objection on broadly pragmatist grounds. (shrink)