Personal Identity: Assessing the Options

David Sanson

2026-09-14

Preliminaries

Agenda

Reminders

  • For today, we are still discussing Chapter 3, “What Makes You You?”
  • Next time, Wednesday, Sep 16th, is our first midterm.
  • Closed book, closed notes, no devices
  • Bring a bluebook and a pen or pencil

How to Prepare for the Midterm

  • Review the slides and your notes (you have been taking notes?)
  • Review the readings
  • I expect you to know the main arguments we have discussed:
    • not just “be vaguely aware of them”
    • be able to actually re-present them, premise by premise.

Review (and some new stuff)

Agenda

Physical Accounts

The Fingerprints Account
\(A\) at time \(t\) is the same person as \(B\) at time \(t^*\) if and only if \(A\) and \(B\) have indistinguishable fingerprints.
The DNA Account
\(A\) at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\) and \(B\) have indistinguishable DNA.
The Same Body Account
\(A\) at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\) has the same body as \(B\).

Psychological Accounts

The Psychological Matching Account
A$ at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\)’s psychological features are exactly the same as \(B\)’s psychological features.
The Psychological Overlap Account
\(A\) at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\)’s psychological features are mostly the same as \(B\)’s psychological features.
The Psychological Descendant Account
\(A\) at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\) is either a psychological ancestor or a psychological descendant of \(B\).

Objections to Same Body Account

The Conjoined Twins Argument

Argument

  • CT1. If the Same Body Account is true, then either Abby and Brittany have different bodies or Abby and Brittany are the same person.
  • CT2. Abby and Brittany have the same body.
  • CT3. Abby and Brittany are not the same person.
  • CT4. So, the Same Body Account is false.

Defense of CT3: not same person

CONJOINED DRAMA

Abby is dating Arie. Brittany is secretly in love with Arie and has always been jealous of their relationship. One night, while Abby is sleeping, Brittany confesses her feelings to Arie, and Arie kisses her. Later, when Abby finds out, she strangles Brittany.

Argument

  1. If Abby and Brittany are one person, then no cheating occurred, and Abby didn’t kill anyone, she just removed one of her two heads.
  2. Cheating did occur, and Abby did kill someone.
  3. So Abby and Brittany are not one person, but two.

The Body Swap Argument

Freaky Friday

On Friday morning, something happens. The person in Anna’s body looks in the mirror in disbelief. “Why,” she thinks to herself, “am I in my teenage daughter’s body?” Meanwhile, the person in Tess’s body looks in the mirror in horror. “Why,” she thinks to herself, “am I in my mom’s middle-aged body?”

  • The person who looks in the mirror and asks, “Why am I in my teenage daughter’s body?” is the Mom, Tess, not the daughter, Anna.
  • So, the person in the teenage body before is Anna, the person in the teenage body after is Tess.
  • So the Same Body Account is false.

The Body Swap Argument

  • Call the person in the mom body on Thursday \(A\).
  • Call the person in the mom body on Friday \(B\).

The Body Swap Argument

  • BS1. \(A\) on Thursday has the same body as \(B\) on Friday.
  • BS2. If \(A\) on Thursday has the same body as \(B\) on Friday and the Same Body Account is true, then \(A\) on Thursday is the same person as \(B\) on Friday.
  • BS3. \(A\) on Thursday is not the same person as \(B\) on Friday.
  • BS4. So, the Same Body Account is false.

The Body Swap Argument and PDA

FREAKY FRIDAY is just one example among many. Can you think of other cases in which a person switches bodies?

An Argument for PDA

Argument

  • PD1. The Psychological Descendant Account fits best with our intuitions about cases.
  • PD2. We should accept the theory that fits best with our intuitions about cases.
  • PD3. So, we should accept the PDA.

Souls?

The Same Soul Account
\(A\) at \(t\) is the same person as \(B\) at \(t^*\) if and only if \(A\) has the same soul as \(B\).

Question: What does the Soul Account say about FREAKY FRIDAY?

What is a Soul?

A soul is an immaterial part of you—traditionally, the part responsible for your ability to think, and the part responsible for your ability to survive death.

The belief in souls is called Dualism—there are two fundamentally different kinds of substance in the world: material and immaterial.

Materialists reject Dualism, holding there are only material substances, and so there are no souls.

Souls and Persons

Blank Slate Reincarnation

The gods run a soul-cleaning shop. It works kind of like a laundry mat. When you die, your soul is sent to the shop, where it waits is turn to go through a soul-cleaning machine. This machine washes the soul clean of all lingering psychological remnants of its previous incarnation. Once clean, the soul is sent back into the world, where it enters the body of a newborn.

You share a part with the newborn. But are you the same person as the newborn?

Souls and Persons

Continuous Reincarnation

When Billy was about to be born, there was a snafu: all the souls were spoken for. One soul was temporarily available: it was needed for Sandy, due to be born a minute later. So they gave Billy that soul, but took it back after one minute. By then, another soul was temporarily available: it was needed for Janice, due to be born a minute later. So they used that soul temporarily for Billy. And so on: Billy has received a new soul every minute of his life.

What does the Same Soul Theory say about Billy? Do you agree?

Bodies, Souls, and PDA

So, to reiterate:

Argument

  • IP1. The Psychological Descendant Account fits best with our intuitions about cases.
  • IP2. We should accept the theory that fits best with our intuitions about cases.
  • IP3. So, we should accept the PDA.

Objections to the Psychological Descendant Account

Amnesia

TOTAL AMNESIA

Jiwoo is stranded on a deserted island. Adding injury to insult, a coconut fell on Jiwoo’s head at noon today, instantly resulting in total amnesia. She can’t remember how she got on the island or anything else about her past. She can’t even remember her own name.

What does the PDA say about this case? Do you agree?

Blackout

TOTAL BLACKOUT

Minjun is stranded on a deserted island. Adding injury to insult, a coconut fell on Minjun’s head at noon today, temporarily knocking him unconscious. While unconscious, he is not dreaming, nor does he have any thoughts or experiences or any physical sensations whatsoever. He is completely blacked out. When he finally awakens hours later, it will feel as if no time has passed.

What does the PDA say about this case? Do you agree?

Fission

DOUBLE TROUBLE

Rachel uploads JoJo’s neural states to Chad’s brain, and also accidentally uploads them to Alex’s brain as well. As a result, both the person with Chad’s original body and the person with Alex’s original body wake up and say ‘My name is JoJo’. Both can tell you all about JoJo’s past; neither can tell you anything about Chad or Alex’s past. She also obliterates JoJo’s original body.

What does the Same Body Account say about this case? What does the Soul Account say? What does the PDA account say?

Fission

The Fission Argument

  • FS1. If the PDA is true, then JoJo is the same person as \(A\) and the same person as \(B\).
  • FS2. If JoJo is the same person as \(A\) and the same person as \(B\), then \(A\) is the same person as \(B\).
  • FS3. So, if the PDA is true, then \(A\) is the same person as \(B\).
  • FS4. \(A\) is not the same person as \(B\).
  • FS5. So the PDA Account is false.

Identity, Death, and Survival

Suppose Jojo agreed to be part of Rachel’s experiment because she had terminal cancer. And suppose Rachel uploaded Jojo’s states to both Chad’s brain and Alex’s brain because the procedure only works half the time, and she wanted to increase Jojo’s chances of survival.

Does this seem right to you?

  1. If Jojo’s states are successfully uploaded to Chad’s brain (but not Alex’s) Jojo survives.
  2. If Jojo’s states are successfully uploaded to Alex’s brain (but not Chads’s) Jojo survives.
  3. If Jojo’s states are successfully uploaded to both Alex and Chad’s brain, Jojo does not survive.

Writing

Agenda

What do you think happens to the original person (Jojo) in fission cases? If you are inclined to endorse the answer given by one of the three accounts discussed—the Same Body Account, the Soul Account, or the Psychological Descendant Account—how would you defend that account against the objections we have seen? If you are inclined to reject all three answers, what do you think we should say about the case?

  • Gather your thoughts (1 minute)
  • Share with someone (3 minutes)
  • Write (10 minutes)
  • Share and discuss