McMullen (1982) attempted in his critique of humanistic psychology to attack its philosophical foundations, taking these to be phenomenology and a doctrine of self determination. In this paper we argue that McMullen has misrepresented the philosophical position of the humanists, then we examine McMullen's own philosophical assumptions. We show that he has assumed the philosophy of empirical realism which we argue is inadequate as a foundation for science. It is shown that it is the inadequacy of McMullen's assumptions which underlies his failure to comprehend the position of the humanists. We then present a version of theoretical realism and show how, in terms of this, a conception of being can be justified which allows for the emergence of hierarchical order. On the basis of this conception of being we reassess the nature of self-determination and motivation, showing how, in opposition to both the empirical realism of McMullen and the philosophical dualism of conventional humanistic psychology, a naturalistic form of humanistic psychology is justified.