Abstract

I argue that joint attention solves the “base problem” as it arises for Schiffer’s and Lewis’s theories of common knowledge. The problem is that an account is needed of the perceptual base of some forms of common knowledge that gets by without itself invoking common knowledge. The paper solves the problem by developing a theory of joint attention as consisting in the exercise of joint know-how involving particular and sometimes distal targets and arguing that certain joint perceivers can always have a minimal form of propositional common knowledge about the location of these targets. On such a view, perceptual common knowledge is based on the experience of a process that is maintained by way of perceivers’ exercise of an object-involving form of joint know-how. Some reductive theories of collective intentionality require that agents’ intentions and subplans are common knowledge (or “out in the open”) between them. For these theories the base problem arises again. The enacted theory of joint attention can solve the problem. The argument is exactly parallel to the common knowledge case. The openness of joint agents’ intentions and meshing subplans is explained by appeal to their practical knowledge of how to maintain the process by way of which they pursue the collective intention. They can then make this knowledge explicit by linguistic communication. When they succeed in communicating knowledge of their meshing subplans as pursued in a joint action context, they necessarily have this knowledge in common. For theories of collective intentionality that include a common knowledge condition, the experience of participating in a perceptually constituted joint action provides the base that renders harmless the regress that otherwise threatens reductive analyses.