More important for Kuhn was the way his account of the context of justification diverged from the standard picture. The functioning of exemplars is intended explicitly to contrast with the operation of rules. The key determinant in the acceptability of a proposed puzzle-solution is its similarity to the paradigmatic puzzle-solutions. Perception of similarity cannot be reduced to rules, and a fortiori cannot be reduced to rules of rationality. This rejection of rules of rationality was one of the factors that led Kuhn’s critics to accuse him of irrationalism—regarding science as irrational. In this respect at least the accusation is wide of the mark. For to deny that some cognitive process is the outcome of applying rules of rationality is not to imply that it is an irrational process: the perception of similarity in appearance between two members of the same family also cannot be reduced to the application of rules of rationality. Kuhn’s innovation in The Structure of Scientific Revolutions was to suggest that a key element in cognition in science operates in the same fashion.