This paper argues that artificial intelligence has failed to address the whole problem of common sense, and that this is the cause of a recent stagnation in the field. The big gap is in common sense---or naive---psychology, our natural human ability to see one another as minds rather than as bodies. This is especially important to artificial intelligence which must eventually enable us humans to see computers not as grey boxes, but as minds. The paper proposes that artificial intelligence study exactly this---what is going on in people's heads that makes them see others as having minds. Introduction: from naive physics to naive psychology Ten years ago, Hayes published the "Second naive physics manifesto" (Hayes, 1985b). Hayes proposed that we "put away childish things by building large-scale formalisations," beginning with "our knowledge of the everyday physical world." He, and others, have since put a lot of effort into developing models of our common sense understanding of ...