The concept of personhood fulfills different roles in different areas of theoretical and practical philosophy. This results in a multitude of competing conceptual definitions of what being a person essentially means. Depending on the concept of person, various characteristics or abilities are identified as criteria for attributing a (descriptive or normative) person status to an individual. Frequently stated are the characteristics of reason, consciousness, self-awareness, purposeful action or the ability of normative orientation.

The question of the criteria for the attribution of personal status must be distinguished from the question of the problem of a person's identity across time: Under which conditions does a person remain the same person over time, i.e. numerically identical to itself, even though it undergoes physical or psychological changes over the course of its existence? Numerical identity is a relational concept from the field of logic and refers to the identity of an object with itself. A numerically identical person is therefore one and the same person and not two persons who are identical in all respects. In philosophical-ethical debates about the conditions of the continuity (persistence) of a person's identity, the concept of diachronic identity is also used in this context, which describes the continuity of the “self” over time (Greek: diá (διά) “through” and chrónos (χρόνος) “time”) despite physical or psychological changes. The fact that an individual fulfills the criteria required to be considered a person at a certain time t1 and that they still fulfill these criteria at a later time t2 does not necessarily mean that they are one and the same person, i.e. that they are numerically identical. It is also possible that it is two numerically different persons.

In order for diachronic identity to be given in the numerical sense, additional conditions must be fulfilled in addition to the general attribution of the status of personhood – exactly what these are is the subject of a long-running and controversial debate in philosophy. There are various approaches that can be used to argue for the (non-)existence of a diachronic personal identity. With regard to the question of which characteristics of a person are decisive for the fact that they can be (re-)identified as one and the same person across time and over various changes, a distinction can be made between psychological and biological views of diachronic personal identity: 

The criterion of psychological connection

According to this criterion, the diachronic personal identity of a person over time is given by the existence of psychological connections between earlier and later mental states. Alongside the memory of past experiences, this includes the translation of current intentions into future actions as well as the long-term persistence of beliefs, attitudes and character traits. These connections structure a person's life over time and ensure their diachronic personal identity in the numerical sense according to this criterion.

Criticisms of the criterion of psychological connection for the attribution of diachronic identity arise from its incompatibility with the general principles of numerical identity: the relation of numerical identity, which stems from logic, does not allow for gradual gradations - two objects or entities are either identical in the numerical sense, if they are actually one and the same object or entity, or they are not. Psychological connections can, however, be gradual, for instance in the density or intensity of memories. As the criterion of psychological connection is therefore subject to some criticism, alternative criteria for the attribution of diachronic personal identity are discussed:

The criterion of psychological continuity

In contrast to the criterion of psychological connection, this criterion focuses on the continuous chain of overlapping psychological connections instead of direct connections between memories. Psychological continuity, and thus identity across time, remains accordingly as long as the chain of psychological connections continues over time, with memories overlapping like the fibers of a thread. Thus, psychological continuity as a criterion for the attribution of diachronic personal identity fulfills the requirements of the non-gradability of numerical identity and is therefore compatible with the prerequisites of numerical identity.

The criterion of physical continuity

Contrary to the two previous criteria, the biological criterion of physical continuity focuses on physical aspects when attributing diachronic personal identity. Thus, the physical continuity of the human organism, or more narrowly the continued existence of a functioning brain, is assumed to be the basis for diachronic personal identity. Here, a person remains the same in the numerical sense as long as their body continues to exist as the same organism or their brain as a functioning organ. Therefore, a person cannot become another person, no matter how profoundly they may change in their psychological characteristics, because such changes do not affect the identity of the biological organism.

Unlike biological theories, psychological theories of the concept of personhood leave room for the possibility of a change in personal identity. This possibility exists when the mental states of a person at different points in time are so radically different 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 example, 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. Another case that has been widely discussed in the field of applied and specifically medical ethics would be advanced stages of dementia. In the context of the debate on enhancement, the question also arises as to whether and to what extent a person's personal identity is influenced by certain biological or biotechnological enhancement methods (see module Literature on enhancement and identity).