Common knowledge is usually defined as a state in which everyone knows that p, everyone knows that everyone knows that p, and so on, ad infinitum. This definition is usually attributed to David Lewis, despite the fact that his own formulation bears no resemblance to common knowledge as it is usually understood. In this paper, I argue that this concept of common knowledge requires revision. Contrary to usual practice, it turns out to be difficult to model formally because existing models fail to distinguish between full-blown common knowledge and merely finite levels of interactive knowledge. Conceptually, the concept is incompatible with Lewis's intended purpose and obscures the explanatory role played by rational choice models. I propose that the concept of common knowledge be brought better into alignment with Lewis's actual formulation. This reconceptualization of common knowledge suggests a greater focus on explanations that make recourse to the cognitive constraints of real-world agents.