John Locke on Personal Identity and Memory

Personal identity requires continuity of mental life, not substance. John Locke argued that even with the same immaterial soul, without psychological connection, we wouldn't be the same person. Contemporary neuroscience confirms this: there's no single brain location for consciousness, and our mental experience is fragmented. We're remarkable ordered collections of thoughts, beliefs, desires, and memories. Derek Parfit extended this by arguing identity over time isn't what matters—psychological continuity matters instead. The question of whether we're the same person as yesterday has no simple answer—it's always partly yes and partly no. This challenges the common-sense view of an enduring 'I' that possesses these things.

The teletransporter thought experiment, presented by Oxford philosopher Derek Parfit, imagines a device that scans your body, disintegrates it, and creates an atom-for-atom replica on Mars. The question is whether this replica is still 'you.' English philosopher John Locke asserts that personal identity is tied to consciousness rather than the substance of a mind, soul, or body. Locke defines the self as a series of repeated acts of consciousness concerning thoughts of the past, future, and present. Scottish philosopher David Hume proposed the bundle theory of self, stating that 'Man is a bundle or collection of different perceptions which succeed one another with an inconceivable rapidity and are in perpetual flux and movement.' Hume argues that these perceptions do not belong to any one thing—the self is identified as the 'togetherness' of these views. Philosopher James Giles interprets this as a 'no-self' theory, suggesting the self does not exist and is instead an evolutionary artefact.

Parfit distinguishes between psychological connectedness and psychological continuity. Psychological connectedness is a direct relation based on degrees and is not transitive—for example, if X remembers most of Y's life and Z remembers most of X's life, it does not follow that X remembers most of Z's life. Psychological continuity, instead, involves overlapping chains of direct psychological relations and is transitive. Parfit argues that connectedness is more important for survival than continuity, and that we should focus on questions of survival rather than identity. He also discusses memory and Locke's account, sketching an account of 'cue memory' to free memory from presuppositions about personal identity: a belief about a past experience that seems like a memory, where someone did have such an experience, and the belief is dependent upon that experience. This allows us to separate memory from the presupposition that it must be connected to individual people.

The paradox challenges the intuition that psychological continuity is sufficient for personal identity—the view associated with John Locke and Derek Parfit that what makes a future person the same person as a present person is the continuity of memories, personality, beliefs, desires, and other psychological characteristics. This view seems plausible for ordinary cases of survival through time—the person who wakes up tomorrow is the same person who went to sleep tonight because there is psychological continuity. But the duplication case shows that psychological continuity cannot be sufficient for identity because identity is one-to-one while psychological continuity can be one-to-many. Multiple future individuals can all be psychologically continuous with a single past individual. If we say that only one of them is the original person, we need a criterion to distinguish that one from the others. No such criterion seems available if all the copies are physically and psychologically identical. One response distinguishes between survival and identity—survival is a looser relation than identity. Survival can occur without identity if there is sufficient continuity and connectedness, and in duplication cases all the copies survive even though none is identical to the original.

Personal identity—the question of what makes a person the same over time—has been debated by philosophers for centuries, with three main theories: (1) Bodily Continuity Theory argues that having the same body maintains identity, but this fails when considering cell replacement or brain transplants; (2) Psychological Continuity Theory, proposed by John Locke, claims identity depends on memory and psychological connectedness, though this implies people with lost memories are no longer the same person; (3) Derek Parfit challenges both, arguing that survival and psychological continuity matter more than strict numerical identity, suggesting we might survive as multiple versions of ourselves without being identical to any single version.

Philosopher Eric Olson's theory of Animalism argues that human beings are essentially biological organisms (human animals) rather than psychological entities, supported by two key arguments: the 'icky animal argument' (if there's a thinking human animal in a chair, and you're the thinking being there, you're that human animal) and the 'fetus argument' (since we were once fetuses lacking mental properties, we cannot be essentially psychological entities but must be essentially human animals).

Animalism is the philosophical theory that personal identity consists in biological continuity rather than psychological continuity; according to this view, we are biological organisms (animals) and our persistence through time depends on the persistence of our biological bodies, not on the preservation of mental states or psychological properties. This theory challenges the traditional psychological continuity view that has dominated personal identity debates for centuries, arguing that accepting psychological continuity views leads to absurd conclusions such as the existence of two separate thinkers (a person and an animal) that could come apart from each other, which violates the intuitive principle that there cannot be two distinct thinkers occupying the same space at the same time.

Animalism challenges the psychological continuity thesis by arguing that personal identity is constituted by biological continuity rather than psychological states. According to animalists, what makes a person who they are is their physical body—the animal organism they constitute. They argue that intuitions about psychological continuity in thought experiments are illusions. Even when we feel a person with a transplanted brain is the same person, animalists claim this is mistaken. The person who wakes up after a brain transplant would be the one with the transplanted body, not the one with the transplanted brain.

Eric Olson argues that human beings are fundamentally animals (animalism), and that being a person is a superficial metaphysical property rather than a fundamental one; he contends that consciousness is the fundamental property for all conscious beings, including non-human animals, and that there are no persistence conditions that apply exclusively to humans, meaning humans share the same persistence conditions as other conscious animals like dogs and cows.

Kennett and Matthews argue that people with dissociative identity disorder are not morally or legally responsible for actions committed by alternate personalities because: (1) They cannot control the sudden emergence of altered states without warning; (2) In altered states, they are under delusion about their identity and moral values, diminishing their capacity to appreciate moral reasons; (3) They have severely diminished autonomy—they cannot make commitments binding across different states. Unlike someone who can plan against future intoxication, DID sufferers cannot implement preventive strategies because alters emerge unpredictably and different personalities cannot influence each other's motivations.

Locke argues personhood is a 'forensic term' tied to legal and ethical issues—specifically reward and punishment. In the Prince and Cobbler case, if the cobbler commits a crime and swaps consciousness with the prince, the person who remembers committing the crime (now in the prince's body) should be punished, not the body with blood on it. Locke applies this to amnesia: if you commit a crime and develop complete amnesia with no consciousness of it, you aren't guilty. This has profound implications for criminal law and moral responsibility.

There are two senses of personal identity: numerical identity (strict identity over time, relevant for legal systems) and moral/practical identity (psychological content like values and concerns). When people say 'I'm no longer the same person' after changing values, they're referring to moral identity, not numerical identity. For moral responsibility, what matters is whether the person still has criticizable psychological states, not whether they are numerically identical to their former self. Legal systems care about numerical identity, but moral responsibility tracks psychological persistence.

Criminal cases involving people who have dissociative identity disorder (DID) have established that a person with DID is still one legal person. Courts have held that criminal or civil liability still attaches to the whole individual even if one alter committed the act and another has no memory of it. In State versus Milligan (1978), the defendant raised DID as a defense. Billy Milligan was charged with kidnapping, rape, and robbery, and experts diagnosed him with DID. The judge found that Billy was not guilty by reason of insanity, and the court accepted the premise that an alter could be legally distinct enough to negate the criminal intent of the host personality. However, since 1978, the law and psychiatry have evolved significantly, and today a person pleading not guilty by reason of insanity must prove that they are unable to distinguish right from wrong or lack control over their actions due to their mental defect.

In Dissociative Identity Disorder (DID), system responsibility means that all parts of a person's system are fundamentally connected and therefore collectively accountable for each other's actions, regardless of how separate or amnesiac the parts may feel; this is supported by structural dissociation theory which states that DID results from a failure to integrate segregated identity states, and legally established through cases like Billy Milligan where courts ruled that multiple personalities do not absolve criminal responsibility since all parts share the same underlying identity and capacity for action.