This chapter’s point of departure is John Neville Keynes’s distinction between positive and normative economics. It relies on this distinction to examine the relationship between positive and normative economics in the works of Karl Marx, Ludwig von Mises and Milton Friedman. It shows that even if the positive economic theories of these economists are very different, all of them have a disparaging view of normative economics, and consider the development of positive economics as the only way of advancing in the resolution of normative disagreements. This chapter challenges this position and shows that the positive economic theories of these economists do not lend support to the normative positions that they espoused. It moreover shows that their pessimistic attitude towards normative economics prevents them from offering rigorous arguments in favour of their positions regarding the reform of public institutions. This critical appraisal aims at underlining the importance of developing theoretical frameworks in normative economics to facilitate the resolution of disagreements concerning politico-economic reforms.