What were the historical conditions of possibility for Structure's naturalism about science and its mode of change? Kuhn's book was remarkable in its time in setting aside the celebration of science and– despite its skepticism about the place in science of Reason, Method, Truth, and Progress—any note of criticism as well. The naturalistic impulse to describe, interpret, and explain, rather than to evaluate and justify, was not a notable feature of commentary about science before Kuhn's work, and I trace this new naturalism to historical changes in the institutional and cultural conditions of science itself in post-World War II America. The possibility of naturalism about the nature of science, I argue, flowed partly from its new institutional security and its strengthened ties with political power.