The concept of personal identity arose arguably within the European intellectual tradition . It has its etymological roots in the colloquial Greek term prosopon and its Latin equivalent persona, signifying “the mask worn in comedy or tragedy” or “the character an actor plays – dramatis personae.” (Chadwick 1981, 193). As early as the sixth century, Boethius (480–524 C.E.), a Latin philosopher and Christian theologian, formulated the concept of personal identity as a synthesis of the Aristotelian concept of substance and the notion of an eternal soul, which early Christian theology had inherited from Neoplatonist philosophy. In the context of the early Christian debates on Christology,  Boethius developed his now famous definition of persona as “individual substance of rational nature” (Lat.: naturae rationabilis individua substantia) (193). The foremost function of this formula was to express that Jesus the Christ, despite his dual nature – divine and human – was unified and one in numero. Nevertheless, the formula additionally identified the “incommunicable quality of the individual within the human species” (194) which functions as the self-identical and individual essence of the human being. Boethius’ definition of the concept “persona” radically differs from its original meaning “mask” in that it, now, denoted that which persisted over time, despite changes and transformations that might occur in its attributes and accidents,  be they physical or psychological in character. It is important to note that Boethius’ usage of “persona” implies unity, endurance, and, most importantly, rationality, thus distinguishing the essence of Mensch-sein not only from the mask that can be arbitrarily and deliberately utilized or discarded by any given actor, but also from inanimate, insensible, and irrational entities. However, it was not until the notion of the individual had developed in the thought of the Italian Renaissance, the Protestant Reformation, and the European Enlightenment that the conception of personal identity as an individual-over-time became the subject of general philosophical debate , involving eminent thinkers such as Gottfried W. Leibniz , John Locke , Hume , Joseph Butler, and Thomas Reid.  By then, the synthesis of Aristotelian and Neoplatonist thought within the Christian theological tradition had given rise to the notion of an individual and enduring core, which clearly demarcates and identifies an individual, human person. During the periods of the Enlightenment and Modernism, this notion of the individual person-overtime was adopted as the general theory of Mensch-sein, underlying most philosophical, psychological, and ethical systems in the West. For all these theories (with the exception of the theories of Hume, Parfit, and some cognitive scientists), the notion of a real, enduring, and conscious agent, namely personal identity, functions as a necessary condition, addressing the issues of ethical responsibility and accountability as well as the continuity of experience. Today, personal identity is defined as persistence-over-time, suggesting that there is one enduring individual which persists through a multitude of different and separate moments in time and which possesses a multitude of experiences.
In some sense , the theory of personal identity emerges from the attempt to explain and conceptualize the sense of continuity characteristic of the existential predicament of the experiential “I.” Awakening to itself, the experiential “I,” that is, the self-conscious agent, finds itself thrown, as Martin Heidegger would say,  into a particular situation and context of historicity. To give an example, every morning I wake up knowing who I am, who my relatives, colleagues, and friends are, what my social function and professional responsibility is, etc. In short, I experience myself continuous and identical with a past self; at the same time, this identity is being reinforced by the community in which I find myself, by the people who meet me, recognize me, and, in some sense create who-I-am in their stories, expectations, and prejudices. The lack of such an experience of identity and the shortage or even negative reinforcement of an assumed identity would create a comical or haunting scenario as exploited in novels such as Joy Fielding’s See Jane Run. This everyday experience of continuity and assumption of identity translates into the contention that the person P exists at, and persists through, diachronically diverse moments such as t1, t2, t3, t4, etc. For the sake of clarity, I would like to identify “P at t1” as P1 and “P at t2” as P2. Any theory of personal identity inquires into the following questions: What is it that continues from t1 to t2? What warrants the attribution of two experiences P1 and P2 to one “person” P? What privileges the relationship between P1 and P2 over the relationship between P1 and, for example, Q2 in the understanding of personal identity? Why is it possible for me to claim identity with the person who graduated from Temple University in 1996 (P2) but not with the person who won the election for the American presidency in the same year (Q2), or with the person who authored the Tao te Ching more than 2000 years ago (R3)?
The colloquial usage of the term “person” is highly ambiguous, signifying both the diachronic person-over-time such as P and individual persons-at-the-moment such as P1 and P2. The notion of the person-over-time designates a set of diachronically separate experiences such as being born at t1, entering high school at t2, and attending college at t3, if and only if at any given time t0 one diachronical, personal unity corresponds to one and only one person – at-the-moment. The term “person-at-the-moment,” on the contrary, signifies one particular experience of a given person-over-time. Assuming the differentiation between person-over-time and person- at-the-moment, Noonan distinguishes between diachronic identity and synchronic identity: the former addresses the relationship between two diachronically disparate person-stages, P1 and P2, while the latter investigates the relationship between one person-stage, P3, and the diachronic stream of experiences called “person,” namely P (Noonan 1989, 104–5). Diachronic identity is expressed in statements of the form “the person who wrote Steppenwolf (P1) and the person who wrote Siddhartha (P2) are identical,” synchronic identity in statements such as “Hermann Hesse (P) is identical to the author of Steppenwolf (P1).” In addition, personal identity does not only require such a diachronic identity relationship between the two persons-at-the-moment P1 and P2 and a synchronic identity relationship between the atemporal person P and person stage P1 but, to be more specific, their exclusive identity in that, at any given time t0, P is synchronically identical to but one person-stage P0 and that person-stage P1 at t1 is diachronically identical with but one P0 at any given time t0. Personal identity thus defined is a clear-cut matter of yes-or-no – tertium non datur.
Thus, the theory of personal identity investigates three central questions: How is it possible to identify a person (myself and others) as an individual human being? How is it possible to distinguish between two individual persons? What guarantees the constancy and identity of an individual person over time? These questions are designed to identify the fundamental characteristics of an individual and to isolate that which makes an individual human being unique. In this sense, the theory of personal identity defines continuity of experience solely as the persistence and preservation of one individual person. As mentioned before, such an enterprise is of utmost importance since its practical implications cover a wide range of problems that include memory, recognition (how is it that I recognize an individual human being whom I have met before?), ethical responsibility and accountability (what are the conceptual conditions to make an individual human being accountable for what s/he has done ten years ago?), and legal problems such as the attribution of property (who is the referent of possessive pronouns?). In short, all these dilemmas hinge on the fundamental question “What are the criteria which identify a human individual-over-time beyond doubt?” At the same time, these questions map out the complexity of the issue, which is reflected in the breadth of approaches to personal identity and the emerging interdisciplinary character of the discourse on personal identity. Recognition evokes the social aspect of human existence, while the delineation of human individuals involves the considerations of communicability of mental content in interpersonal interaction and the exchangeability of human organs in transplants as well as the clarification and definition of “content” and “delineation.” In addition, concepts such as “identity,” “substance,” and “continuity” have metaphysical and, for the most part, logical implications. Most of all, however, it has to be examined how the various theories of personal identity reflect the human experience of identity and difference, endurance and transformation. Finally, and this point is often overlooked within the discourse on personal identity, the quest for personal identity suggests two methodologies, a first-person-approach and a third-person-approach: The former inquires “How do I define myself?” and “How can I identify what is intrinsically me?” while the latter defines and recognizes an other’s personal identity from the outside.