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 two previous paper I wrote: one titled “Coase’s Parable,” which was published in the Mercer Law Review in 2023; the other, “Modelling the Coase Theorem,” published in the European Journal of Legal Studies in 2013. 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.”