Psychogenic depersonalization is an altered mental state consisting of an unusual discontinuity in the phenomenological perception of personal being; the individual is engulfed by feelings of unreality, self-detachment and unfamiliarity in which the self is felt to lack subjective perspective and the intuitive feeling of personal embodiment. A new sub-feature of depersonalization is delineated. 'Prosthesis' consists in the thought that the thinker is a 'mere thing'. It is a subjectively realized sense of the specific and objective 'thingness' of the particular (...) object thought about. I show that prosthesis is an important cognitive feature of depersonalization, and may be psychologically connected with the tendency of depersonalized individuals to report 'philosophical' types of thinking. Indeed, several philosophical issues concerning the identity of the self appear to have been enhanced by prosthesis experiences. Thus, far more efficient than William James's experimental attempts to uncover philosophical truths under the influence of nitrous oxide intoxication, prosthesis may be a safe and recommended experience for philosophers. The history of depersonalization theories is presented from Krishaber to Freud, and the main approaches to prosthesis criticized. Finally, a fresh approach to psychogenic depersonalization is outlined on the basis of certain cognitive similarities with visual agnosia. This paper may be understood as continuing the Jamesian tradition 'experimental abnormal psychology', that is, of examining extraordinary mental states with an eye to their philosophical implications. (shrink)