The origins of post-Keynesian economics are notoriously diverse: Smith, Ricardo, Marx, Marshall, Keynes, Kalecki and Sraffa are all perceived as important influences. Moreover, various relatively distinct branches of post-Keynesianism are often identified as emerging from these roots. Although certain general characteristics can be identified including an opposition to orthodox approaches and an emphasis upon the significance of institutions, the complexity of human agency and importance of uncertainty, numerous competing substantive positions are developed. A question that arises from the existence of such distinct approaches within post-Keynesianism is whether they can all be interpreted as mutually consistent and belonging to a coherent whole. In recent years this has become a somewhat controversial issue.