  In the period between Knowledge and Human Interests and The Theory of Communicative Action, Habermas began to develop a distinctive method for elaborating the relationship between a theoretical social science of modern societies, on the one hand, and the normative and philosophical basis for critique, on the other. Following Horkheimer's definition of Critical Theory, Habermas pursued three aims in his attempt to combine social science and philosophical analysis: it must be at once explanatory, practical, and normative. This meant that philosophy could not, as it did for Kant, become the sole basis for normative reflection. Rather, Habermas argued, adequate critique requires a thoroughgoing cooperation between philosophy and social science. This sort of analysis is characteristic of Legitimation Crisis (1975; German ed., 1973e), in which Habermas analyzes the modern state as subject to endemic crises, which arise from the fact that the state cannot simultaneously meet the demands for rational problem solving, democracy, and cultural identity. Here the social science to which Habermas appeals is more sociological and functional. Similarly, in this work and in Communication and the Evolution of Society (1979), Habermas begins to develop a distinctive conception of rational reconstruction, which models societal development as a learning process. In these works, Habermas begins to incorporate the results of developmental psychology, which aligns stages of development with changes in the kinds of reasons that the maturing individual considers acceptable. Analogously, societies develop through similar changes in the rational basis of legitimacy on the collective level. At this point in his theorizing, Habermas's appropriation of the social sciences has become methodologically and theoretically pluralistic: on his view, a critical social theory is not distinctive in light of endorsing some particular theory or method but as uniting normative and empirical inquiry.