Contemporary sociologies often defines themselves as "actor-centered" sociologies. In them, society appears less as a reality sui generis that constrains the individuals than as something constructed day-by-day by the various social actors. But who are these actors? In what way are the human beings actors? That is a question that is hardly ever asked. The fact that human beings must be endowed with a capacity for social action, a capacity for historicity or, in other words, a capacity to be the actors or authors of their own histories, is one of the major postulates of contemporary sociology. However, it is possible to go beyond this postulate and attempt to account for what it is that makes human beings capable of historicity. Because this is not self-evident. A certain number of pathologies, both in the psychiatric and the neurological fields, seem to affect this ability of humans to be the authors of their own history as actors and potential interlocutors in a relationship with others. This, at least, is the hypothesis proposed by Jean Gagnepain which has gathered support from clinical experience and has resulted in the concept of "person". As Gagnepain explains, if a human being is a social actor who is capable of historicity then it is first and foremost because he or she is a person. By this, he means that humans possess a specific mental capacity to create social bonds, the functioning of which is partially revealed through the clinical study of perversions and psychoses as well as of certain cerebral lesions. The aim of the present book is to summarize and assess our current knowledge of the person understood in this sense. Though written by a single author, it is underpinned by the work performed by an entire team, most of whose members were tutored directly by Jean Gagnepain himself.