Koestler developed his concept of the 'holon' to take on three key issues that he believed to be facing the social sciences in the post-war generation. First, he found the need for some model that could bring together and integrate the reductionist and mechanistic perspectives of the 'scientific' and behavioural psychologies with the holistic and humanistic perspectives of the Freudian, Gestalt, and Rogerian psychologies. Second, he recognized the significance of evolutionary processes in the social sciences and sought to provide some theoretical system that could apply evolutionary formulations to both fields. Lastly, he wished to construct a model of human social systems that was equally familiar in analyzing the micro-level of individuality and the macro-level of collectivity; a basic explanatory model that could relate across the span of human activity and involvement.