The paper provides a panoramic view on Keynes’s way of reasoning in economics, with a view to highlight the radical “difference” of his thinking with respect to both scholars of his times and contemporary economists. We argue that Keynes was un-modern in the 1930s as he is (and appears) un-modern today; but also that in this lies his modernity. Keynes is modern in trying to transform economics into a discipline of complexity; still, he is profoundly un-modern when discussing the conditions that economics must meet in order to achieve this result. The paper evidences in his strong commitment to avoiding any reductionist approach to the complexity of socio-economic systems the undisputed greatness of his thought.