Market rationality is a central point of contention among economists, investment professionals and politicians. Some argue that markets (including financial ones) make everyone better off; they are the ultimate generator of win-win situations. Narratives here range from the reasonable to the quasi-religious. Their detractors point out the many instances where markets fail to generate these win-win situations, with narratives ranging from the reasonable to the dystopic “corporations are evil” rant. As the recent events pitting retail investors against hedge fund managers showed us, there is a pressing need to understand the nature of market rationality – lest we misunderstand it.