Ash Catton

PhD Candidate, Clinical Psychology Student

John Locke's Account of Personal Identity

October 13, 2017


1      Introduction

Personal Identity (PI) has been the source of ongoing debate among philosophers for centuries and the matter is by no means settled. The 17th Century philosopher John Locke sought to provide an early account of diachronic identity, also referred to as identity-over-time. Essentially, he aimed to explain what makes an adult the same person from one moment in the time to the next. Locke argued that it was the continued existence of one’s psychological processes that held a person’s diachronic identity together. His insistence on psychological continuity (PC) as both necessary and sufficient condition for PI to obtain is not without a share of significant challenges. In this paper, I will argue that Locke’s argument is unable to account for modern technological innovations that may exist on the horizon, which renders his argument somewhat incomplete. However, most damaging to Locke is his refusal to consider the material body as a necessary condition for PI. I argue that this provides tension around matters of death, provides bizarre consequences on matters of personal responsibility, and results in a logical contradiction where identifying consciousness is concerned. Lastly, I lend support for the Constitution Theory of PI which manages to overcome these objections to Locke’s account.

2      Locke’s Account of Personal Identity

2.1     Groundwork
It is important to lay some foundations upon which we can build Locke’s account. When we say that x is identical to y, we have two possible meanings. Either we say that x has the same material composition as y, or, if some time has passed between observations of x and y, that there is some other common property that these things share. The latter account, diachronic identity, is what Locke is addressing in his Essay Concerning Human Understanding, and is also what I will mean when I refer to Personal Identity (PI). Specifically, the task that Locke has set himself is to answer the question of what we mean when we say that x in the year 2017 is the same person as x in 2000. For Locke, the passage of time in this statement indicates that the persistence of some common property over time is a necessary qualification.

Locke’s first claim is that the identity of a thing is not the same as its material composition.[1] He does this by providing examples of an oak tree and a watch, where the identity of these things is dependent on the continued existence of those constituent parts necessary to keep the whole object functional. Trees can shed bark, leaves, and branches, and watches can have parts added and removed which shows that something other than mere continuity of material composition is necessary to identify these things. As Locke later notes, one could cut the limb off a person and their identity is not adversely affected.[2]

The purpose of these examples is to show that identity must be referring to what Locke describes as ‘unity of substance’ or a system of processes specific to that thing.[3] And as processes require time, it is evident that continuity must form part of a coherent account of identity over time. Thus, continuity of some characteristic process must be a necessary condition and Locke believes that the characteristic substance of persons is their consciousness.

2.2    Consciousness
By positing consciousness as the substance of persons, Locke manages to distinguish the person from the body they inhabit, as the human body entails its own characteristic material substance. Consequently, the expressions same person and same woman are not synonymous as they refer to things defined by different criteria.[4] As a person would be defined as a thinking being with capacities for reflection and reasoning, we must supplement our continuity criterion with the substance responsible for it, namely consciousness. Locke maintains that consciousness is the sole factor that unites the actions of a person over time.[5]

       Psychological Continuity
An initial objection arises from Locke’s use of the term ‘consciousness’ to describe the processes characteristic of persons which requires us to be more precise with our language. A modern prima facie interpretation of the term could suggest that events like sleep, or failure to recall the experience of an event, causes disruption for our continuity criterion. Thus, to more accurately describe the system of processes that define a person it would be appropriate to refer to Psychological Continuity (PC). As Locke defines persons as the conscious, thinking thing that is aware, feels both emotion and sensation, and is concerned for itself, we can consider PC as encompassing the same phenomenon that Locke referred to as ‘consciousness’.[6] Furthermore, as psychological processes continue to persist throughout our lives, defining consciousness in this way overcomes the potential misunderstandings arising from sleep and the inability to recall single events. Thus, we can state Locke’s criterion for PI accordingly:

x is the same person as x* iff x is psychologically continuous with x*

3      Problems for Locke

In this section, we examine the objections that Locke addresses in his work but with a modern perspective to see if PC is both a necessary and sufficient condition for PI to obtain.

3.1     Same Person, Different Substance
Locke considers an objection that could arise from the possibility of one person existing across more than one substance which provides psychological processes.[7] For the objection to work, he states that we would need to know what the substance was, and whether psychological processes could be transferred to it (to maintain continuity). He dismisses this possibility as very unlikely as he believes God would not be so cruel as to extract consciousness from one entity and implant it into another. The historical context of Locke’s essay affords him this dismissal; however, this scenario may be more likely in the present century.

Advocates of the Transhumanist movement often refer to a concept called uploading. This is a theoretical process whereby consciousness is transferred from a biological vessel (i.e. a brain) to a computer.[8] Even though such a concept may remain forever hypothetical, our advances in technology mean that this is a possibility which poses some challenges for Locke’s account of PI.[9] Locke’s insistence on a linear continuity of psychological processes, means that any disruption in the flow of consciousness that may arise from the transfer to a computer, forces him to state that death occurs for the person who originally possessed said consciousness. If Locke’s argument is to survive the transhumanists claims, he would need to revise his ‘all or nothing’ position and come up with an alternative that allows for some flexibility in discourse pertaining to PI.

Derek Parfit’s ‘survivability’ solution may benefit Locke’s position by replacing the ‘one-one’ relations of consciousness with degrees of existence. Parfit comes to this conclusion by highlighting the impracticalities of psychological continuity simpliciter. He asks us to imagine a person’s brain being split at the corpus callosum and one of the halves being implanted into another body.[10] The challenge is to consider where the person’s identity lies given that they have been split into two co-existing entities of consciousness. Locke would have to concede that either the original person died, or they existed as one of the two surviving conscious entities. Parfit furthers his argument by asking us to imagine the two conscious streams reconciling back into the same skull. That Locke would be tying himself in knots trying to subsume this scenario under his account of PI suggests that new terminology is needed to accurately describe psychological continuity. 

Survivability can be visualised as a family tree, where consciousness is able to survive instances of uploading and split-brain co-existence. What is measured is not the linear trajectory of a single consciousness but the degree to which it continues to exist after the events described. I do not believe that altering the nature of the relation between person and PC poses as much of a problem for Locke. Rather, it seems to complement his account by strengthening it against twenty-first-century objections. 

3.2     Different Persons, Same Substance
The next objection that Locke considers is one that involves different persons existing under the same unifying substance. Again, with historical context, Locke foresees the objection arising from someone like a Buddhist or Hindu who may consider the unifying substance as being the soul, and the different persons being manifestations of the same soul.[11] He considers the possibility of a past life in ancient Troy and rejects the idea that this would be the same person as himself. He strengthens this position by stating that one must have memories of these past lives to qualify for PC.[12] In other words, there must be some mental continuity between these lives to qualify as being the same person. This is an understandable move for Locke given that memory is a fundamental component of our psychological processes. Thus, continuity of soul is neither sufficient nor necessary for PI to obtain. However, Locke’s historical limitations mean that he was unable to foresee examples where different persons could manifest from the same conscious substance.

       Dissociative Identity Disorder (DID)
The definition of DID (formerly Multiple Personality Disorder) specifies a “…discontinuity of identity and memory” with the presence of at least two personality states coupled with episodes of amnesia.[13] As an example we can assume that x and y are distinct personality states that originate from the same conscious substance.[14] As there is no psychological continuity between x and y, Locke would be forced to admit that they are different persons. Jolley suggests that Locke may elaborate on this by stating that they would be the same woman but different persons due to the differing criteria for each.[15] However, for the physicalist, they are both ultimately united by the same substance – the brain. Even if one did not subscribe to physicalism there are still some challenges that arise.

There seems to be some tension here whereby Locke would have to state that x dies every time y manifests given that there is no continuity of psychological relations between each personality state. However, if x retained memories of her actions when she manifested again, he would have to say that she had not died permanently. This is another instance where Locke may benefit from some flexibility. However, even with Parfit’s survivability, Williams argued that it is easier to refer to the manifestation of these personalities as arising from the same person to avoid having to figure out which behaviour is to be attributed to which personality.[16] This position fits more closely to the modern understanding of DID where each personality is a fractured aspect of a single person.[17]

DID poses another problem for Locke’s account. If x commits a crime but y is manifest at the time police arrive on the scene, who is responsible for the act? Jolley might defend Locke and suggest that the ‘woman’ be arrested, however this might mean that y is also wrongly arrested and convicted for something x did. This problem is not limited to DID, as Mackie notes it is also applicable to acts committed while sufficiently intoxicated as to cause amnesia.[18] Furthermore, Locke’s insistence of the relation between memory and persons means that both persons x and y are, despite occupying the same body, both entitled to vote separately and even marry different spouses.[19]

3.3     Whose Mental State is it Anyway?
Our last problem for Locke involves his premise that PI obtains only by psychological continuity, and not by any relation to the body that a person’s consciousness inhabits. He illustrates this point with a thought experiment in which a prince and a cobbler switch consciousness and wake up in each other’s bodies.[20] The point is to remind us that only consciousness provides identity, but this point is troublesome. As Lowe highlights, Locke’s dependence on mental states requires consciousness to have its own criteria for identity which cannot involve either the material body or the soul given his rejection of either construct as a necessary condition.[21] This leaves Locke with the option of anchoring consciousness on the identity of the person to which it belongs, thus rendering a vicious infinite regress. By throwing the body out with the bathwater, Locke has removed a necessary condition for Personal Identity.

4      Salvaging Psychological Continuity

Locke’s characterisation of human persons as intelligent self-concerned beings is correct, and his inclusion of all the psychological processes that provide intelligence and self-awareness seems to be a necessary criterion of personal identity. However, if we are to have a coherent account of PI that addresses the objections which arise from the rejection of material composition, then we will need an account that combines both. Furthermore, as some of the objections I have presented have relied on a physicalist answer to the mind-body question, this would need to be accommodated to avoid those same charges. The Constitution View, presented by Lynne Rudder Baker, seems to be the best fit to these problems. Her argument is that personal identity is an epiphenomenal product of the entire composition of the human body, which includes the brain.[22] Baker elaborates on this by showing that Michelangelo’s David statue is a product of, but not identical to, the marble that constitutes it.[23] Thus, on the Constitution View, a person is identified by their capacity to have a first person (conscious) perspective.[24] In this way, we are still able to refer to Psychological Continuity as a necessary condition but where it now incorporates the biological entity which maintains it in much the same way that the identity of Locke’s oak tree necessarily incorporates those material elements that allow its continued existence. This account navigates around the upload problem by stating that the death of the person is synonymous with the death of those material capacities that afford their first-person perspective. Any artificial replication of that perspective will be precisely that: a replication.

5      Concluding Remarks

Locke’s account of PI is robust and rightly draws attention to the continuity of psychological processes as a necessary condition for diachronic PI to obtain. For x and x* to be the same person, it is evident that some continuity of mental processes is necessary to unify them. However, PC is not a sufficient condition for PI. Dissociative Identity Disorder shows how Locke would have to tie himself in knots trying to differentiate between ‘persons’ residing in the same body, and would also have troubles when it came to the allocation of rights for each of these persons. Further, his rejection of any phenomenon outside of consciousness invites several problems which the account cannot sustain. By including PC as a necessary, but not sufficient condition, we can supplement Locke’s account by anchoring PC into the substance that creates and maintains it, which the Constitution View is able to provide.



Works Cited

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders Dsm-5.  Arlington, VA: American Psychiatric Association, 2013.
Baker, Lynne Rudder. Persons and Bodies.  Cambridge: Cambridge University Press, 2000.
———. "What Am I?". Philosophy and Phenomenological Research 59, no. 1 (1999): 151-59.
Bostrom, Nick. "The Transhumanist Faq." World Transhumanist Association,
Jolley, Nicholas. Locke: His Philosophical Thought.  Oxford: Oxford University Press, 1999.
Locke, John. An Essay Concerning Human Understanding.  East Sussex, United Kingdom: Delphi Publishing, 2017. 1689.
Lowe, E. J. Locke on Human Understanding.  London: Routledge, 1995.
Mackie, John L. Problems from Locke.  USA: Oxford University Press, 1976.
Parfit, Derek. "Personal Identity." The Philosophical Review 80, no. 1 (1971): 3-27.
Proudfood, Diane, and Jack Copeland. "Our Posthuman Future." The Philosophers Magazine, 2012.
Sinnott-Armstrong, Walter, and Stephen Behnke. "Responsibility in Cases of Multiple Personality Disorder." Philosophical Perspectives 14 (2000): 301-23.
Williams, B. A. O. "Personal Identity and Individuation." Proceedings of the Aristotelian Society, New Series 57 (1956/1957): 229-52.


[1] John Locke, An Essay Concerning Human Understanding (East Sussex, United Kingdom: Delphi Publishing, 2017), II. xxvii. 3-5.[2] Ibid., II. xxvii. 11.[3] Ibid., II. xxvii. 6.[4] Ibid., II. xxvii. 7.[5] Ibid., II. xxvii. 16.[6] Ibid., II. xxvii. 17.[7] Ibid., II. xxvii. 13.[8] Nick Bostrom, "The Transhumanist Faq," World Transhumanist Association,[9] Diane Proudfood and Jack Copeland, "Our Posthuman Future," The Philosophers Magazine2012.[10] Derek Parfit, "Personal Identity," The Philosophical Review 80, no. 1 (1971): 5-11.[11] Locke, II. xxvii. 14.[12] Ibid., II. xxvii. 20.[13] American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders Dsm-5 (Arlington, VA: American Psychiatric Association, 2013), 291.[14] This paper assumes argues from physicalist position whereby consciousness and the brain are taken to be the same substance. Locke explicitly expressed agnosticism in the mind-body debate.[15] Nicholas Jolley, Locke: His Philosophical Thought (Oxford: Oxford University Press, 1999), 111.[16] B. A. O. Williams, "Personal Identity and Individuation," Proceedings of the Aristotelian Society, New Series 57 (1956/1957): 248.[17] American Psychiatric Association, 292.[18] John L. Mackie, Problems from Locke (USA: Oxford University Press, 1976), 183.[19] Walter Sinnott-Armstrong and Stephen Behnke, "Responsibility in Cases of Multiple Personality Disorder," Philosophical Perspectives 14 (2000): 305.[20] Locke, II. xxvii. 15.[21] E. J. Lowe, Locke on Human Understanding (London: Routledge, 1995), 115-16.[22] Lynne Rudder Baker, "What Am I?," Philosophy and Phenomenological Research 59, no. 1 (1999): 154-55.[23] Persons and Bodies (Cambridge: Cambridge University Press, 2000), 29-30.[24] Ibid., 91.