Personal identity


John Locke (1632-1704) is generally credited with being the first western philosopher to consider the question of the diachronic identity of persons. What does it mean to be a particular human being?

This isn’t a numerical and classificatory problem in the sense that you might ask how many people there are in a population. To answer that, you would implicitly answer another question, what does it mean to be a human being, and then, based on this conceptual model, count the number of people present. This is the synchronic aspect to the question of individuation. It applies not just to human beings but to any entity which can be individuated.

The diachronic identity of persons is a different question. What is it that leads us to think we have just the one person though time? It can’t be identity as such because change is constant. Few if any components that go to make up a person last a lifetime. We are continually in the process of acquiring a history. Locke’s answer was that it was continuity of consciousness that functioned to secure the identity of persons through time.

This seems intuitively on the right path, but it raises some questions. In the way that Locke intends it – as a forensic concept which binds together foresight, actions and consequences and supports ideas of morality, law and justice – it really applies only to human beings. However, every natural entity has diachronic identity of some kind, if only transient. Plants and animals are individuated but continuity of consciousness in this sense doesn’t apply to them.

Is continuity of consciousness, therefore, one among multiple possible modes of diachronic identity or is the situation that continuity of consciousness is something additional that applies to human beings alone but is layered on top of other forms of diachronic identity shared with living organisms generally.

This turns out to be a much more complicated question than it might at first appear. I realised while putting this note together that there are actually two different concepts of individuals involved. One might be called individuality, which can be contrasted with type. The distinction is that the type, say human being, is an abstraction that ignores the differences between individuals and focuses on what is common and shared. The individuality of a human being is what is particular to the individual, the characteristics that are either within the expected variations or are accidental to the type.

The second idea might be called individuation. Individuation is the separation of entities into individuals as opposed to being undifferentiated stuffs. Human beings are both individualised and individuated whereas the leaves on a tree are individuated but not particularly individualised. Material substances are only accidentally individuated, like the puddles of water that form after a downpour but quickly evaporate. Single water molecules, in contrast, are intransigently individuated but entirely lacking in individuality.


For something to have diachronic identity there must be both some partitioning of reality into separate individual entities and, at the same time, a degree of integration which places a limit on the partitioning. Integration limits divisibility. Furthermore, the evolution of the individual entity must happen in a way that maintains both the separation and the integration throughout its life-cycle, from the time at which the components are originally brought together until the time at which they are finally dispersed.

There is a spectacular diversity of structures in nature but we can simplify the picture by looking at them at a very abstract level. At a high level of abstraction, four types of individual entity can be identified: individual beings, individual things, individual systems and individual samples of stuff. In addition to these categories, entities can be classified into those which are characterised by a distribution of control and those characterised by a distribution of function.

By samples of stuff I am thinking of what we usually mean by material substances such as water, oxygen, limestone and so on. We don’t usually think of stuff as being individuated but by thinking in terms of samples we can see that it is. We never come into contact with water as such, only samples of water, even if the samples are vast and long-lived, as they are in the case of oceans, or small and transient, as in the case of a puddle. In this category I would place collections of small and highly undifferentiated objects such as grains of sand and seeds, which are usually treated as stuffs even though they composed of individual objects.

The basic distinction between a substance and a system is the degree of homogeneity of the components. Like all such distinctions, the difference is graduated rather than sharp. Whereas stuff is homogeneous, systems are characterised by the diversity of components. In an eco-system such a forest or a lagoon each type of component evolves according to its own organising and ordering principles.

Systems are characterised by a distribution of control. In a system each component evolves according to its own organising principles. There is no coordinating or controlling centre. The consequence is that the system as a whole evolves through interaction, adaptation and accommodation of the components.

Entities in which there is a distribution of function, on the other hand, require some kind of central coordination and control. In nature, distribution of function is a characteristic of living organisms, a consequence of an apparent design for survival. Each component plays a role in the functioning of the whole. Living organisms are homeostatic entities:

There is, however, a more serious objection to the idea that biological structures are self-organised. Structures such as [a splash pattern] differ from biological structures in one crucial respect: they may be complicated, but they are not adapted to ensure their own survival and reproduction. It is this apparent design for survival that distinguishes living structures (and human artifacts) from non-living ones. An eye is designed for seeing: a vortex merely exists.

John Maynard-Smith: Shaping Life

The distinction I am drawing between beings and things is the distinction between conscious and non-conscious entities. In both plants and animals there is a distribution of function but most animals have a whole organism neural system which co-ordinates the internal functioning of the organisms and manages the way in which it navigates the external world. This higher level of functional distribution requires centralised communication, coordination and control.

This analysis doesn’t imply that any entity is one thing or another. These distinctions are graduated and most entities are complicated and contain multiple modes of functioning. A human being is, to one way of thinking, a collection of systems, some local, some whole organism. Each system operates with some level of autonomy within the whole. At the same time, there is a coordinating and communicating centre managed through the neural system.

Change in all entities happens both internally, in a reconfiguration of the existing components, and externally, with an exchange of components across the entity boundary; components entering from or departing to the external world. Entities in which there is a distribution of control can more easily support an exchange of components with the outside world. Such entities can support higher levels of turnover of components because there is much less interdependency of components. Entities in which there is a distribution of function, on the other hand, will be much more vulnerable to exchange of components because of the integrated functional dependency.

If there is change, how can there be identity? There are two ways of thinking about identity through change. Firstly, there is identity in terms of composition, which might be called compositional identity. Secondly, there is identity in terms of process and we can call this process identity.

Process identity comes into play once components can enter and leave across the entity boundary. Rivers makes a good model for process identity. The water in the river is constantly flowing though, but we can reasonably say that it is the same river because although in terms of composition it is constantly changing, it nevertheless flows from the same source to same destination between the same banks.

Most entities, including human beings, have primarily process identity rather than component identity. Process identity can include changes of form as well as changes of contents and configuration.

Process identity stretches but not infinitely. Most entities have a defined life-cycle. The components are gathered together to form an entity, retained or recycled during its life-cycle and, eventually dispersed when the entity fails. The process that binds a set of components to form an individual entity in the first place may not be the same process that maintains the entity throughout its evolution.

Understanding the origin, structure and evolution of the diversity of natural entities is the subject matter of natural science and it is an infinitely intricate and complex task.  But at a very abstract level it is possible to simplify considerably and focus on these three very abstract aspects to the question of the diachronic identity, namely: separation, integration and persistence.

In this analysis, there are three principal break points: the division between natural systems and living organisms, the division within living organisms between living things and living beings, and the distinction within living beings between consciously aware and introspectively self-aware beings. That is, the distinction between natural eco-systems and environments and living organisms, the distinction between plants and animals, and the distinction between animals and human beings.


We live as individuals and we only ever encounter other individuals. For this reason, the abstract conceptual models and general principles of science and philosophy do not easily apply to individuals.

There are two aspects to being an individual entity, individuation and individuality. Individuation is a question of partitioning and integration, whereas individuality is a question of difference. Individual entities are always individuated but not necessarily significantly individualised. The leaves on a tree are individuated but not particularly individualised. A person, on the other hand, is both individuated and individualised.

This has a consequence for our ability to understand and explain the world. We think and speak in terms of abstract types and general principles. Types are conceptual model and individual entities are understood as instances of the type. How successfully we understand and explain the world therefore depends to a large extent on how far we can capture significant information in the type. A type will abstract both from the totality of an entity and its situatedness.

Individuated entities occupy a unique location in space and time and therefore exist in a unique relation to everything else. This means they have a unique history, even when they are not particularly diversified. In terms of attributes, an individuated entity may be close to type and have very few individualising attributes but the uniqueness of its situatedness places a limit on the extent to which explanation and understanding can be achieved simply by considering the type. As well as being uniquely situated, individualised entities are also diversified, less close to type, in terms of attributes. Individuality places an additional limit on what can be explained by considering an entity as an instance of the type.

Intuitively, understanding and explanation should vary by the degree of individuation and the degree of individuality. We would expect it to be most achievable with material substances and least achievable with human beings, with natural systems, plants and animals somewhere in between, and this is indeed what we find.

Living organisms differ from material substances and natural systems to the degree that they are adapted to ensure their own survival and reproduction. There is no mechanism that sustains a river, holding it together, in the way that a tree is sustained as a living organism.

In the case of an animal, that individuation is more pervasive, as the functioning of the whole organism is monitored, coordinated and communicated. There is not only a distribution of function but a centralised surveillance capability implemented in the neural system to monitor the way in which the entity is functioning, co-ordinate information about the state of the organism and communicate adjustments to maintain its homeostatic functioning.

In the case of a human being, that individuation becomes self-conscious. We are aware of our own individuation and individuality. This generates a tension between our living and our thinking, because we live as individual human beings but conceptualise ourselves as instances of the type human being.

If we think and speak in terms of types, but live as individuals, how well can we really understand our own existence? Do the limitations of understanding stem primarily from the diversified individuality of human beings or from the deeper layers of individuation? My feeling is this: the individuality of human beings flows from the deeper layers of individuation. Conscious conformity to type will limit individuality but not individuation. However, diversity will tend to flow naturally from individuation. We are different because we are self-consciously aware of ourselves as individuals and therefore of the openness of the paths available to us.


In human beings, the markers of an animal’s identity, separation and integration, are augmented to a further level, self-awareness.

In normal circumstances, when we are awake and alert, without fuss or deliberation, the images that flow in the mind have a perspective – ours. We spontaneously recognise ourselves as the subjects of mental experiences…we each appreciate mental contents in a distinct perspective, mine or your…

The term “consciousness” applies to the very natural but distinctive kind of mental state described by the above traits. That mental state allows its owner to be the private experiencer of the world around and, just as important, to experience aspects of his or her own being…

It is tempting to simply talk about “subjectivity” and leave behind the term “consciousness” and the distractions it tends to cause. We should resist the temptation, because the term “consciousness” conveys an additional and important component of conscious states: integrated experience, which consists of placing mental contents into a more or less unified multidimensional panorama.

Antonio Damasio: The Strange Order of Things

The concept of subjectivity can be misleading. Subjectivity in this context is a consequence of individuation rather than individuality. An individual’s point-of-view is subjective because it is a consequence of separation rather than because it is particular or different from other individuals.

This implication can be obscured when subjectivity is contrasted with objectivity. Objectivity implies that an experience is the same for all observers and therefore subjectivity can be interpreted as a source of error or bias. Science strives to be objective in this sense. However, if what is individual is grounded in individuation, then there is no such dichotomy.

If subjectivity is the form of separation in human beings, consciousness is the basis of integration. Self-conscious subjectivity in turn creates the platform for language, cultural acquisition and cultural transmission. Antonio Damasio continues:

In conclusion, subjectivity and integrated experience are the critical components of consciousness…subjectivity and consciousness are essential enablers of the cultural mind. In the absence of subjectivity, nothing matters; in the absence of some degree of integrated experience, the reflection and discernment that are required for creativity are not possible.

The picture of the diachronic identity of persons that emerges from this analysis emphasises both the continuity and the discontinuity between the diachronic identity of persons and the diachronic identity of entities generally. The forensic concept that Locke proposes, which forms the basis of law and justice and morality, is a possibility created by introspection grounded in language and the cultural mind. It Is an evolution of the diachronic identity of beings which in turn is an evolution of the homeostatic functioning of living organisms, the apparent design for survival that John Maynard-Smith notes.

Damasio, A. (2018). The Strange Order of Things. New York: Pantheon Books.

Maynard Smith, J. (1998). Shaping Life. London: Weidenfeld & Nicholson.