Scientific discovery has long been explained in terms of theory, data, and little else. We propose a new approach to scientific discovery in which tools play a central role by suggesting themselves as scientific theories, by way of what we call the tools-to-theories heuristic of scientific discovery. In this article, we extend our previous analysis of statistical tools that became theories of mind to the computer and its impact on psychological theorizing. We first show how a conceptual separation of intelligence and calculation in the early 19th century made mechanical computation, and later the electronic computer, conceivable. We next show how in this century, when computers finally became standard laboratory tools, the computer was proposed—and eventually adopted—as a model of mind. Thus, we travel the full circle from mind to computer and back.