The concept of personhood fulfills very different roles in different areas of theoretical and practical philosophy. This results in a multitude of competing definitions of what being a person essentially means. With regard to the follow-up question of which characteristics of a person are decisive for them being (re-)identified as one and the same over time and through various changes, a distinction can be made in particular between biological and psychological interpretations of personal identity: Biological theories on personhood answer the question of the conditions of the persistence of a person with recourse to the human being as which this person manifests themselves. It follows that a person cannot become another in the numerical sense (cf. module on Numerical identity), no matter how profoundly they may change in their psychological characteristics, because such changes do not affect the identity of the biological organism. 

Psychological theories on personhood, on the other hand, postulate that a certain form of mental continuity must exist between the states of a person at different times in order for them to be considered states of one and the same numerical person. Depending on the theory, the identity-preserving continuity is defined more closely by a certain minimum degree of memory or character stability, for example. Unlike biological ones, psychological theories on the concept of personhood leave room for the possibility of a change of personal identity. This possibility exists when the mental states of a person differ so radically at different times that it does not seem plausible to attribute these states to the same person. In the case of memory, a change of personal identity in the numerical sense could manifest itself, for instance, in the form of complete retrograde amnesia, i.e. in situations in which a person has lost all memory of experiences prior to a traumatic event.