The author describes and examines Ian Hacking’s approach to historical ontology and its application to psychological description. Historical ontology is concerned with the study of phenomena that come in and out of being. Phenomena under psychological description not only are historical, but also are particularly susceptible to what Hacking calls dynamic nominalism. Dynamic nominalism characterizes the ways our descriptive practices of naming interact with things named. More precisely, in describing ourselves psychologically, we humans are uniquely capable of reacting to such self-descriptions in ways that can constitute or reconstitute a relation with ourselves. Humans come to define and act toward themselves under psychological descriptions and, in the process, form and alter the kinds of persons they are. Furthermore, it is suggested that an indispensible precondition of historical ontology, and all that is entailed by it, is a uniquely human agency that not only is responsive to psychological descriptions, but also is self-interpreting and self-determining. The author concludes with some impressions about historical ontology and psychological description and their implications for psychology.