This article asks why the analogy between humans and computers was understood by cognitive psychologists to mean that "minds exist and that it is our job as psychologists to study them". Earlier psychologists, such as Clark Hull, used analogies between humans and complex machines such as telephone switchboards to defend a rigorous behaviorism. How, then, did the computer metaphor of mind come to be seen as the root concept underlying a paradigm shift from behaviorism to cognitivism? To answer this question, this article examines the life and work of George A. Miller, one of the most prominent of a generation of psychologists who began their careers as "good behaviorists" but later came to embrace cognitivism.