The chapter starts with a murder trial hypothetical where the defendant claims they are not the same person as the murderer due to changes over time.
The argument is based on the idea that the person has changed their preferences (Led Zeppelin to Todd Rundgren), physical attributes (appendix removal), and age (25 to 30).
The court would reject this argument because it confuses qualitative and numerical identity.
Qualitative sameness: Similarity in qualities or characteristics. When someone changes their appearance or beliefs, they are qualitatively different.
Numerical sameness: Being one and the same entity. Represented by the equals sign (=) in mathematics (e.g., 2+2=4).
We are numerically the same person we were as babies, despite vast qualitative differences.
Punishment is only justified if the person being punished is numerically identical to the wrongdoer.
Numerical identity is crucial for emotions like anticipation, regret, and remorse, which we only feel for ourselves.
Personal identity: What makes a person the same person over time.
The Question of Personal Identity
The concept of personal identity is dramatized through an example of God bringing you back to life 500 years after death.
The question arises: how can God ensure it is truly you in the future, given that your body will have decomposed and its matter scattered?
Creating a similar person from new matter is not sufficient; it needs to be you.
The same issue arises with ordinary change over time, like looking at baby pictures and identifying that baby as you.
Philosophical reflection extends to the identity of objects other than persons (electrons, trees, bicycles, nations), but personal identity is uniquely tied to emotions and self-interest.
Reconstituting a physically similar person from new matter is not enough for personal identity; it's just qualitative similarity.
Using the same matter is also insufficient. The matter that makes up your body might have been part of someone else in the past.
Sameness of matter is neither sufficient nor necessary for personal identity.
People constantly change their matter through ingestion, excretion, hair cuts, shedding skin, and grafts, yet they remain the same person.
The Soul
Some philosophers and religious thinkers believe the soul is the key to personal identity.
The soul is the person's psychological essence, a nonphysical entity where thoughts and feelings reside.
The soul persists through physical changes and even after bodily death.
Your soul is what makes you you.
God can bring you back to life by inserting your soul into a new body.
However, there's no solid evidence for the existence of souls.
The argument that souls are needed to explain thoughts and feelings has been undermined by neuroscience.
Brain injuries cause psychological damage, and specific brain areas are linked to particular psychological effects.
It's plausible that mentality resides in the brain, making the soul unnecessary.
Brain theorists are beginning to explain how the brain produces thought through complex neuron interactions, while soul theorists lack a comparable explanation.
If souls have no smaller parts (like neurons), it's hard to see how they could perform the complex task of thinking.
Spatiotemporal Continuity and the Case of the Prince and the Cobbler
Spatiotemporal continuity: Bases personal identity on natural phenomena.
Analogy: Identifying a baseball over time by tracking its continuous path from the pitcher's hand to the catcher's mitt.
Although usually unnecessary for identifying people due to distinct appearances, this method is helpful for identifying identical twins.
Spatiotemporal continuity is a practical guide to personal identity.
Philosophically, we seek the essence of personal identity beyond mere practical guides.
Example: determining whether a man is a bachelor by looking at his messy apartment is only a superficial observation.
The true essence of being a bachelor is: unmarried male.
The true essence of being gold is: atomic number 79.
Philosophical accounts of essence must hold true in all possible circumstances.
The spatiotemporal continuity theory states that spatiotemporal continuity is the essence of personal identity.
Refinement: Persons are numerically identical if and only if they are spatiotemporally continuous via a series of persons.
John Locke's example: A prince and a cobbler swap psychologies.
In Locke's original story, the souls of the prince and the cobbler are swapped. Let's change this story: The brains of the prince and the cobbler are altered, without any transfer of soul or matter, by an evil scientist.
The person in the cobbler's body has the prince's memories and desires and vice versa.
The spatiotemporal continuity theory says that the person in the cobbler's body is still the cobbler.
Locke disagrees; he believes the person in the cobbler's body is now the prince.
Argument: If the prince committed a crime and swapped minds to avoid punishment, it would be a miscarriage of justice to punish the person in the prince's body (who now has the cobbler's mind).
The person in the cobbler's body (with the prince's memories) should be punished, implying he is the prince.
Psychological Continuity and the Problem of Duplication
Locke uses the prince and cobbler example to argue for psychological continuity as the basis of personal identity.
Psychological continuity theory: A past person is numerically identical to a future person if the future person has the past person's memories, character traits, etc., regardless of spatiotemporal continuity.
Locke's theory implies that the person in the cobbler's body is the prince and should be punished for the prince's crimes.
Challenge: The duplication problem presented by Bernard Williams.
Evil scientist scenario: Charles and Robert both receive the psychology of Guy Fawkes.
According to Locke, both Charles and Robert would be identical to Guy Fawkes.
However, this leads to the absurdity that Charles and Robert are identical to each other.
If x = 4 and y = 4, then x = y.
In just the same way, if we know that Charles = Fawkes and Robert = Fawkes then we can conclude that Charles = Robert.
Though qualitatively similar, Charles and Robert are numerically distinct people.
Duplication problem: What happens when psychological continuity is duplicated (or triplicated, etc.)?
Williams favors spatiotemporal continuity to avoid the duplication problem.
People can survive the loss of parts, like amputations, which introduces spatiotemporal discontinuity.
Sufficient spatiotemporal continuity is needed to account for changes in parts while retaining the same person.
Imagine cancer only in the right half of your body, including the right brain hemisphere. Doctors separate your body, discard the cancerous half, and give you prosthetics.
The person after the operation is still the same person, even with significant spatiotemporal discontinuity.
Even half the body's continuity can be sufficient for personal identity.
Modified scenario: Cancer in both brain hemispheres. Doctors divide the body in two before radiation treatment to improve the odds of success.
Unexpected outcome: both hemispheres are cured, resulting in two people, each with one of your original hemispheres.
Each half-person is sufficiently spatiotemporally continuous with you.
The spatiotemporal continuity theory implies that you are identical to both new persons, which again leads to the absurd conclusion that these two new persons are identical to each other.
Both psychological and spatiotemporal continuity theories face the duplication problem.
Theories state a single original person can be continuous with two successor persons. Therefore, the original person is identical to each successor person, implying the successors are identical to each other.
Radical Solutions to the Problem of Duplication
One solution could be appealing to souls, but there is no evidence to support this claim.
Restating scientific theories: Personal identity is nonbranching continuity.
Continuity usually does not branch, but the duplication example is branching.
The restated theory implies that there is no personal identity in such cases. Therefore, Charles nor Robert is identical to Guy Fawkes, and you do not survive the double-transplant operation.
Derek Parfit challenges the assumption that personal identity is important.
Parfit says that personal identity is nonbranching continuity, which means that continuity rarely branches.
For Parfit, Psychological continuity is what's really important.
Parfit challenges the assumption that stopping existing is bad.
Radical solution: Reconsider the assumption that personal identity is numerical identity.
The duplication argument states that if personal identity holds between the original person and each successor person, we get the absurd result that the successor persons are the same person as each other. However, this only follows if personal identity is numerical identity, the same notion that the equals sign (=) expresses in mathematics.
Perhaps personal identity is never really numerical identity.
Perhaps all change really does result in a numerically distinct person. If so, we would not need to say that branching destroys personal identity.
Even this stricter notion would be looser than numerical identity.