That is the title of the paper, available here via SSRN, that I will be presenting this weekend at the Winter Institute for the History and Philosophy of Economics at the University of Austin (UATX). My paper is an updated version of a previous paper I wrote titled “Coase’s Parable,” which was published in the Mercer Law Review in 2023. In my updated paper, I revisit Ronald Coase’s cattle trespass hypothetical and explore the origins of his counter-intuitive insight that harms are a reciprocal problem, an idea that I now call “Coase’s axiom.”