This paper examines extant ways of classifying varieties of psychological externalism and argues that they imply a hitherto unrecognized distinction between shallow and deep externalism. The difference is between starting points: shallowly externalist hypotheses begin with the attribution of psychological states to individuals, just as individualistic hypotheses do, whereas deeply externalistic hypotheses begin with agent-environment interaction as the basis of cognitive processes and attribute psychological states to individuals as necessary for such interaction. The over-arching aim is to show how deep (...) externalism works and what its implications are for psychological and philosophical theorizing. (shrink)