Much of what we say is never said aloud. It occurs only silently, as inner speech. We chastise, congratulate, joke, and generate endless commentary, all without making a sound. This distinctively human ability to create public language in the privacy of our own minds-to, in a sense, "hear" ourselves talking when no one else can-is no less remarkable for its familiarity. And yet, until recently, inner speech remained at the periphery of philosophical...