The logic of contemparary public policy research is postpositivists' critical Mutiplisrn. This author attempts to analyze how and why postpostivitists systematically attacked on the positivists' theory of knowledge that was dominant until 20 tears ago in most philosoply of science, and on the social reforms in the 1960s and 1970s aimed at ameliorationg social problem. Secondly, the theories and forms of multiplism, and urges students of public policy studies ti construct a more a useful theory of multiplism. The analyses of these questions help us to understand the methodological basis of policy research: mutipism.