Psychological theories of diachronic identity

David W. Agler

The Pennsylvania State University

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Same psychology theory

Same psychology

If A is a person at time t1 and B is a person at t2, then B is the same person as A iff B at t2 has the same psychology as A at time t1.

Problems

Vague: what does “same psychology” mean?
False: No core part of your psychology that stays the same

Simple memory theory

Simple memory theory

If \(A\) is a person at time \(t_1\) and \(B\) is a person at \(t_2\), then \(B\) is the same person as A iff \(B\) at \(t_2\) has the memories of the experiences of \(A\) at time \(t_1\).

flowchart RL
    A[A at t1]
    B[B at t2]
    B e1@== remembers ==> A
    e1@{ animate: true }

Problem:

We are not always remembering our prior selves
We don’t have perfect memories of the past.

Revisions

Occurrent memory

An occurrent memory is a memory someone is actively having.

Non-occurrent memory

A non-occurrent (dispositional) memory is a memory someone is capable of having.

Factual memory

A factual memory is a recollection of a fact about the world.

Procedural memory

A procedural memory is a memory of how to do something.

Personal (episodic) memory

A procedural memory is a memory about (1) an experience of an event but also (2) subjective information derived from the experience of that event.

What kind of memory?

Tek is actively remembering what he did yesterday.

Occurrent memory
Dispositional memory

What kind of memory?

Tek could remember his locker combination but right now he is solving a Rubik’s cube.

Occurrent memory
Dispositional memory

What kind of memory?

Tek remembers that Harrisburg is the capital of Pennsylvania.

Factual memory
Procedural memory
Personal (episodic) memory

What kind of memory?

Tek remembers that how excited he was when he was in Harrisburg.

Factual memory
Procedural memory
Personal (episodic) memory

What kind of memory?

Tek is solving the Rubik’s cube.

Factual memory
Procedural memory
Personal (episodic) memory

Apparent vs genuine memory

Genuine (strong) memory

Person \(A\) genuinely remembers doing event \(E\) only if \(A\) actually did \(E\).

Apparently (weak) memory

Person \(A\) apparently remembers doing event \(E\) only if \(A\) believes \(A\) did \(E\).

Identity and apparent memory

The claim that survival requires genuine memory is circular.

\(A=B\) iff \(B\) genuinely remembers what \(A\) did.
\(B\) genuinely remembers what \(A\) did iff \(B=A\)

flowchart RL
    R[A remembers B]
    I["A=B"]
    R <==> I

We need something that tells us when \(A=B\) but does not assume \(A=B\).

Example: A=B iff A and B have same body
A&B having the same body does not assume A=B

Identity and apparent memory

\(B\) having an apparent personal memory of \(A\) does not assume \(A=B\).

Tek at t1 is the same person as Tek at t2 iff Tek at t2 believes he remembers Tek’s conscious experience at t1.
David in 2026 is the same as David in 2013 who won a race in Tyrone, PA iff David believes himself to remember the conscious experience of winning the race.

Simple memory theory (revised)

If \(A\) is a person at time \(t_1\) and \(B\) is a person at \(t_2\), then \(B\) is the same person as A iff \(B\) at \(t_2\) has the apparent personal memories of \(A\) at time \(t_1\).

Problem: Transitivity

\(C\) remembers \(B\) and \(B\) remembers \(A\), but
Therefore, \(C=B\) and \(B=A\)
By transitivity \(C=A\)
But, \(C\) does not remember \(A\).
Therefore, \(C\neq A\). Contradiction with (3)

Reid’s objection

Exercise

Create your own version of Reid’s famous objection.

Modified memory theory

Modified memory theory (MMT)

If \(A\) is a person at time \(t_1\) and \(B\) is a person at \(t_3\), then \(B\) is the same person as \(A\) iff either (i) \(B\) at \(t_3\) has an apparent non-occurrent memory of an experience of \(A\) at time \(t_1\) or (ii) there is some person \(C\) at \(t_2\) such that (a) \(B\) has an apparent non-occurrent memory of the experiences of \(C\) and (b) \(C\) has an apparent non-occurrent memory of the experiences of \(A\).

flowchart RL
    A(A at t1)
    C(C at t2)
    B(B at t3)
    B e1@== remembers ==> A
    B e2@== remembers ==> C
    C e3@== remembers ==> A
    e1@{ animate: true }
    e2@{ animate: true }
    e3@{ animate: true }

Gaps problem

There are periods of time when you have no apparent non-occurrent memory of the experiences of person \(A\) but you are person \(A\).

Blackout drunk
Temporary brain injury
Knocked unconscious (smelling salts not working)

Implausible results

Suppose you awoke one day and could remember everything the biblical Noah experienced.
Those memories were as clear in your mind as what you were doing 1s.
MMT would say You = Noah

Charles is Guy Fawkes?

Suppose Charles in 2026 claims to be Guy Fawkes (1570-1606)

Charles can apparently remember the life of Guy Fawkes
Charles’s testimony is consistent with what we know about GF
Charles provides additional proof, e.g., hidden secrets

Liz is Guy Fawkes?

Suppose Liz claims to be Guy Fawkes (1570-1606)

Liz can apparently remember the life of Guy Fawkes
Liz’s testimony is consistent with what we know about GF
Liz provides additional proof, e.g., hidden secrets

Fission Problem

Charles = GF
Liz = GF
Liz \(\neq\) Charles. Contradiction

Exercise

Create your own version of the Guy Fawkes fission objection.

Psychological connectedness

Psychological connectedness theory

Let’s try to fix the gaps problem with a new theory.

Psychological Connectedness Theory

If \(A\) is a person at time \(t_1\) and \(B\) is a person at \(t_3\), then \(B\) is the same person as \(A\) iff either

\(B\) at \(t_2\) has an apparent, non-occurrent memory of \(A\) at \(t_3\), or
there is some person \(C\) at \(t_2\) (between \(t_1\) and \(t_3\)) such that \(B\) has an apparent, non-occurrent memory of \(C\) and \(C\) has an apparent, non-occurrent memory of \(A\), or
there is psychological continuity between \(B\) and \(A\).

Psychological connectedness

By psychological connectedness, we mean some weighted sum of psychological similarities or connections:

belief continuity: \(A\) believes at \(t_1\) that Liverpool FC is the best team ever, so does \(B\) at \(t_2\).
Trait carryover: \(A\) has sense of humor \(F\) at t1, \(B\) has sense of humor \(F\) at t2.
Desire chains: \(A\) wants to sign up a marathon at t1, and \(B\) has this same desire at t2.
Information retention: Person \(A\) knows X at t1 and \(B\) knows X at t2.
belief / emotion formation: \(A\) has experience \(E\) at \(t_1\) (victim of attack) and \(B\) is fearful of the attacker at \(t_2\).

Example: Mary to Liverpool

Mary in Chicago wants to go to Liverpool
She buys ticket.
Blacks out as she enters the plane.
Arrives in Liverpool.

Psychological Connectedness Theory

Intuitively, Mary in Liverpool is the same person as the person in the plane as the person in Chicago even though there is a memory gap.

Psych conn = best theory?

flowchart RL
    A(A at t1)
    B(B at t2)
    C(C at t3)
    C(D at t4)
    C e1@== remembers ==> B
    B e2@== remembers ==> A
    B e2a@== psy conn ==> A
    C e3@== psy conn ==> B
    D e4@== psy conn ==> C
    e1@{ animate: true }
    e2@{ animate: true }
    e2a@{ animate: true }
    e3@{ animate: true }
    e4@{ animate: true }

Fixes the memory gaps problem facing the modified memory theory
Gives the intuitively correct result for brain transplants
Gives the intuitively correct result for persons with dicephalus
Gives the intuitively correct result for artificial brain replacement

Objections to the psychological connectedness theory

Teletransportation

Suppose you are offered a free trip in the teletransportation device.

flowchart LR
    A[Tek at t1]
    B[Teletransporter]
    C(Tek? at t2?)
    A e1@== enters ==> B
    B e2@== exits ==> C
    e1@{ animate: true }
    e2@{ animate: true }

Teletransportation details

Scan of brain and body
Data sent to intended destination
Fall asleep
Body reconstructed in intended destination
Old body destroyed after new body wakes up.

Psychologically

Stand in machine
Close eyes
Wake up in new location
Retain all of your memories
Feels like closing your eyes for a split second.

What does the theory say?

Psychological connection between \(A\) and \(B\), so \(A=B\).
Psy theory: you would survive teletransportation.

Exercise

Would you use the teletransporter?

Fission

flowchart LR
    C[Tek at Chicago]
    T[Teletransporter]
    P(Tek? in Paris)
    V(Tek? in Vegas)
    C e1@== enters ==> T
    T e2@== exits ==> P
    T e3@== exits ==> V
    e1@{ animate: true }
    e2@{ animate: true }
    e3@{ animate: true }

Tek-P = Tek-C
Tek-V = Tek-C
Therefore, Tek-P = Tek-V (transitivity)
But, Tek-P\(\neq\) Tek-V.

Teletransportation device

Exercise

Draw a scenario where the teletransportation device results in fission, e.g., who is entering the machine, where are they going?
What would be a practical consequence of fission, e.g., money is split in bank account?

Five solutions

Pick one: Tek-P is the real one.
Pick one: Tek-V is the real one.
Neither one: neither Tek-P nor Tek-V are Tek-C
Both: Tek-P = Tek-V = Tek-C
No answer.

Pick one

Both of these solutions are completely arbitrary.
No reason to pick one over the other.

Neither one

Two reasons to accept.

R1: Teletransportation is death.
R2: People survive teletransportation but not when fission occurs.

R1: Death machine?

The problem with saying teletransportation (TT) is death is that there is no psychological difference between these two cases:

Tek-C falls asleep in airport and wakes up
Tek-C enters TT and wakes up in Paris.

Psy theory needs to say what is the psychological difference. Physical theory might say that teletransportation results in death because of non-continuity of body.

R2: If fission, no identity

non-branching condition

If \(A\) is a person at time \(t_1\) and \(B\) is a person at \(t_2\), then \(B=A\) iff there is psychological continuity between \(B\) and \(A\) and there is no other person \(C\) who is psychologically continuous with \(A\) at \(t_2\).

Problems with R2

Ad hoc: no reason why you shouldn’t survive fission.
Violates only x and y principle

Only x and y principle

only x and y principle

Whether a later individual, \(y\), is numerical identical to an earlier individual, \(x\), only depends upon facts about \(x\) and \(y\) and the relations between them (it does not depend upon facts about some other individual \(c\))..

Examples

If \(x\) is Superman at \(t_1\) is \(y\) is Clark Kent at \(t_2\), then Superman = Clark Kent depends upon these two individuals, not some other individual “John”.
Whether my body survive the replacement of my arm depends my body at two separate stages, not what is going on in some other part of the world.

Both - Problems

A person cannot be wholly located in two distinct places at once (Paris and Vegas).
A person knows what they are thinking / experiencing, but Tek-P and Tek-V don’t know what the other is thinking / experiencing.
If Tek-P dies, Tek-V survives. Strange to say that part of Tek died.
Tek-P and Tek-V might fail to recognize themselves.

No answer

“If all the possible answers are implausible, it is hard to decide which of them is true, and hard even to keep the belief that one of them must be true. If we give up this belief, as I think we should, these problems disappear.”

Many unsolved problems:

Vagueness problem (tallest short person)
Maybe God’s existence
Millennium Prize Problems

Problem 1: Ostrich

“If all the possible answers are implausible, it is hard to decide which of them is true, and hard even to keep the belief that one of them must be true. If we give up this belief, as I think we should, these problems disappear.”

Yes, all problems “go away” if you just ignore them.

Problem 2: Unknowable vs. unknown

No solution vs. solution exists but it is not known.

Comte (1830s): Humans will never know the makeup of stars (later discovered via spectroscopy)
Mach (atoms): Cannot be known because they cannot be observed
Gleason’s theorem: math theorem thought to be unprovable constructively, later there was a constructive proof (Richman, Bridges)

Three more solutions

Gradual separation: both, then …
Reject reduplication
Reject psychological approach

Gradual separation

Some people say that

at \(t_2\) Tek-C = Tek-P = Tek-V, but
at \(t_3\) Tek-P \(\neq\) Tek-V

OK, but

at \(t_3\), is Tek-C = Tek-P?
at \(t_3\), is Tek-C = Tek-V?+

Reject reduplication

Claim is that reduplication is impossible.

Practically impossible? Maybe.
Technologically impossible? No.
Metaphysically impossible? No.

Reject psychological approach

Order we discussed theories:

flowchart LR
    S(Soul)
    Phy[Physical]
    Psy[Psychological]
    S ==> Phy
    Phy ==> Psy

Where to go now?

flowchart LR
    S(Soul)
    Phy[Physical]
    Psy[Psychological]
    New["New theory?"]
    Up["Give up"]
    S ==> Phy
    Phy ==> Psy
    Psy ==> Phy
    Psy ==> S
    Psy ==> New
    Psy ==> Up

Exercise

Is there a solution to the teletransportation problem?

Psychological theories of diachronic identity

Slide commands

Same psychology theory

Simple memory theory

Revisions

What kind of memory?

What kind of memory?

What kind of memory?

What kind of memory?

What kind of memory?

Apparent vs genuine memory

Identity and apparent memory

Identity and apparent memory

Problem: Transitivity

Reid’s objection

Modified memory theory

Gaps problem

Implausible results

Charles is Guy Fawkes?

Liz is Guy Fawkes?

Fission Problem

Psychological connectedness

Psychological connectedness theory

Psychological connectedness

Example: Mary to Liverpool

Psych conn = best theory?

Objections to the psychological connectedness theory

Teletransportation

Teletransportation details

Psychologically

What does the theory say?

Fission

Teletransportation device

Five solutions

Pick one

Neither one

R1: Death machine?

R2: If fission, no identity

Problems with R2

Only x and y principle

Both - Problems

No answer

Problem 1: Ostrich

Problem 2: Unknowable vs. unknown

Three more solutions

Gradual separation

Reject reduplication

Reject psychological approach

Solution