The common ground of a conversation corresponds roughly to what is taken to be out in the open or public among those talking. The notion is at the center of quite a lot of theorizing about linguistic communication. But while formal models of common ground abound in pragmatics, less attention has been paid to its foundations. Can we say what a common ground is in more fundamental terms? What configuration of states of mind at the individual level makes for common ground at the group level? The standard answers to this question in the literature explain common ground in terms of (common) belief or (common) acceptance, and leave knowledge entirely out of the story. I’ll argue that this is a mistake. Common ground has a fundamentally epistemic foundation, one in common knowledge. I’ll defend this view in part by motivating a thesis beyond the boundaries of pragmatics, viz.: common knowledge can explain coordinated action with a depth that common belief cannot match.
