Physicalist theories of diachronic identity

David W. Agler

The Pennsylvania State University

Physicalist Theories

Same body theory

Same body theory

Person \(A\) at \(t_1\) is one and the same person as person \(B\) at \(t_2\) (\(A=B\)) iff \(A\) and \(B\) have the same body.

You at t1 and me at t2 are different people because we have different bodies.
David at t2 is the same as David at t1 because he has the same body

Objection: Same body?

What do we mean by “having the same body”?

Not: qualitatively identical bodies because bodies change
Not: some singular part that stays absolutely constant

Common response

Some argue that there is some physical part of you that remains unchanged throughout your life.

Two problems

Typically, that part has nothing to do with your diachronic pesonal identity
That part does change

Common responses: Neurons

Scientists thought the number of neurons were fixed.
In the 1960-1980 it was discovered that new neurons formed in the hippocampus

Death of neurons

While neurons are longest living cells of body, they die as we age, are injured, or diseased (Parksinon’s Huntington’s, Alzheimer’s).

Change

So, the number of neurons change but we survive (increase, decrease)

They Change

Change at the atomic level: take in Oxygen, glucose, amino acids, push out waste
Molecules breakdown and are rebuilt
Axon terminals contain synapses w/ receptors. Learning increases the number of receptors.

Common responses: Eyes

Size of the eye might not change much, but …

They undergo protein turnover, DNA repair, membrane replacement, metabolic changes, molecular maintenance, etc.
Irrelevant: You would still be you if you lost your eyes

Eye Lenses

The lens of the eye has minimal change:

Many lens fiber cells persist throughout your lifetime
New lens cell fibers are added ontop of old ones
Proteins inside these cells are long-lived

Eye Lens

Proteins inside the eyes undergo various modifications, e.g., oxidation, molecular change
Even if they didn’t change, you would still be you if you lost the lenses of your eyes (intraocular lens (IOL) replacement)

Common responses: Tooth enamel

Most of your tooth enamel is just minerals (no living cells)
The cells that produce tooth enamel (ameloblasts) are lost after you gain a tooth.

Image of tooth enamel

Objections: Tooth enamel

You didn’t become you once you gained your adult teeth.
You don’t die after you lose the enamel on your teeth

Having the same body

“Having the same body” \(\neq\) unchanging material part
Having the same body must accommodate change.

Theory needs

Physicalist theory needs to specify what kinds of change a body can undergo and stay the same body.

Exercise

Survival?

Which of the following could you survive?

replacement of an old skin cell with a new skin cell (part replacement)
loss of an arm (loss of part)
replacement of an arm with a robotic arm
Gradual replacement of 49% of your human cells through natural growth and decay
Rapid replacement of 100% of your human cells with different human cells
Gradual Replacement of 99% of your human cells and parts with donkey cells and parts
Complete destruction of your body and construction of “new you” with totally new parts

Biological theory

Three conditions

Spatiotemporal continuity
Same kind of thing
Gradual change

Spatiotemporal continuity

Intuition: body \(A\) at t1 and body \(B\) at t2 are the same bodies if there is a continuous physical connection between \(A\) to \(B\).
We can draw a path in spacetime.

Implication: If a person materialized in this room and looked exactly like you, they would be an imposter.

Same kind of thing

You have a new body if you change the kind of body you have.

Human \(\to\) Dolphin. Death
Human \(\to\) Donkey. Death

Gradual change

Intuition that you cannot survive rapid, full body replacement.

All of my body parts replaced in 1s \(\to\) New Body. Death.

Biological theory

Same body theory

Person \(A\) at \(t_1\) is one and the same person as person \(B\) at \(t_2\) (\(A=B\)) iff

there is a continuous physical connection between \(A\) and \(B\) (spatiotemporal continuity),
\(A\) and \(B\) are of the same kind (same species), and
any change from A to be \(B\) is gradual in nature.

All three are necessary requirements
Together they are sufficient.

Argument from Language

P1: Our language tracks reality.
P2: When we talk about ourselves as persisting, we express it in terms of our body persisting.
C: Therefore, we are just our bodies persisting through time.

Examples

If I walk to the store, I am talking about my body walking to the store.
If you ask me where I was yesterday, I give you the location of my body.

Argument from Grounding

Our physical states are more fundamental than our psychological states (the latter are grounded in the former)
Proof: For any psychological state (e.g., pain) there is a corresponding physical state (brain state), but not necessarily vice versa.
Therefore, fundamentally, we are nothing but our bodies existing through time.

Simple Fetus Argument

P1: You are a person now.
P2: You were once a fetus.
IC: Therefore, the person you are now is numerically identical to that fetus.
P3: There is no psychological connection between you and the fetus.
P4: There is a biological connection between you and the fetus.
C: Therefore, you are just your body.

Simple Fetus Argument (Continued)

Fetus question

Were you ever a fetus?

Mad Scientist Argument

You are kidnapped
Your body has your psychology removed
It is replaced with a new psychology
Your body is now tortured.

Mad Scientist (Continued)

Mad Scientist Question

Which events cause you personal concern (not just concern for someone else or personal property)?

Continued

You are kidnapped
Your body has your psychology removed
It is replaced with a new psychology
Your body is now tortured.

If step (4) is causing you personal concern, it indicates that you consider yourself to be your body.

Arguments against the biological theory

Rapid Change

Bio theory says change must be gradual.

Rapid change 1

I can survive these:

Tattoos and piercings
Cosmetic implants: breast / calf implants, subdermal horns, facial fillers
Alterations: tongue bifurcation, earlobe stretching, neck elongation, tooth filings, ear pointing (elfin ears)
Scarification or branding
Organ Transplants: kidney transplant, pace maker
Neural implants: cochlear implants, neuralink

Rapid change 2

I cannot survive rapid replace of every organ (heart, liver, kidney, brain), bone, and fluids (blood)

Response

I survive iff:

the change is gradual, OR
when the change is rapid, there is no significant modification to the “essential” body parts

Question

What body parts are essential (absolutely necessary) to survival?

Problem: Brain transplants

I can survive the loss of an arm.
I can survive the replacement of an arm with a new arm.

Can I survive the replacement of any single body part?

Brain transplants

Suppose Bob and Liz swap brains.
Where does Bob go?
Where does Liz go?

Answers

Answer 1: Only one organ was replaced. Bob is still in Bob’s body. He just has a new brain.
Answer 2: Brain priority. Bob is now in Liz’s body. He has a new body.
Answer 3: Both dead.

Exercise

Draw a diagram where you swap brains with someone. Where do you go?

Dicephalic parapagus

Dicephalic parapagus (two heads, one torso) is extremely rare (1 in 100,000)
Even more rare is survival after birth.
Abigail Hensel and Brittany Hensel (separate and shared control of the body)
Elementary school teachers

Same body theory

Bio theory: there is one body. Therefore, one person.
There are clearly two people.
Therefore, the bio theory is wrong.

Same brain theory

Same brain theory

Person \(A\) at \(t_1\) is one and the same person as person \(B\) at \(t_2\) (\(A=B\)) iff they have the same brain. Two brains \(C\) and \(D\) are the same brain iff:

there is a continuous physical connection between \(C\) and \(D\) (spatiotemporal continuity),
\(C\) and \(D\) are brains of the same kind (e.g., human brains)
any change from \(C\) to be \(D\) is gradual in nature

The good

No worries about rapid replacement of arms, legs, tattoos, etc. (brain is essential body part)
We go where our brain goes (solves transplant problem)
Dicephalic parapagus involves two people (not one)

Problem: Fetus problem

Bio theory was attractive because it makes us identical to the human fetus.

You are now a person
You are identical to anyone who has your brain.
Early fetus does not have a brain.
Therefore, you never were a fetus.

Problem: Hemispherectomy

You could survive a full brain transplant
People survive corpus callosotomies (separation of the two hemispheres)
People survive hemispherectomies (removal of one of the hemispheres)

Yearly checkup

You get a routine checkup
Disease in your left hemisphere.

Dr. Tex recommedation: hemispherectomy and transplantation of left hemisphere into new body.

Survival?

Would you survive?

Fission

Tek = LH
Tek = RH
Therefore, LH = RH (transitivity)
But, LH \(\neq\) RH. Contradiction!

Brain modification

Survival?

Suppose you need a small part of your brain replaced with an artificial part. You will die without it. If you had the operation, would you still be you?

Artificial brain

One condition of brain theory: \(A\) = brain \(B\) iff the same kind of brain (human).

End of the process, you have an artifical brain (not human brain).
Conclusion: you don’t need to have the same kind of brain.

Spatiotemporal continuity

Your brain has an infection. You will die in days.
Scan your current brain in every detail.
You receive anesthesia and fall asleep

New Brain

You wake up and remember:

Being told you had a brain infection
Being told you would die
Having your brain scanned
Receiving anesthesia and falling asleep.

New Brain

But, you have a new brain!

Conclusion: Survival does not require spatiotemporal contintuity of the brain.

Body swapping

Mad scientist takes two people: Alice and Bob.
He plans to swap the minds of Alice and Bob.

So,

The mind of Alice is put into Bob’s body.
The mind of Bob is put into Alice’s body.

Body swapping torture

Before the procedure, Bob is asked:

After the procedure, which body would you like to be tortured and which body would you like to be set free and receive ten million dollars?

Presumably, Bob will say:

“I want the Bob-body to be tortured and I want the Alice-body to receive the freedom and money.”

Body swapping torture

This suggests that you go where your mind goes.
Survival does not require spatiotemporal continuity.

Same brain theory

Same brain theory

Person \(A\) at \(t_1\) is one and the same person as person \(B\) at \(t_2\) (\(A=B\)) iff they have the same brain. Two brains \(C\) and \(D\) are the same brain iff:

there is a continuous physical connection between \(C\) and \(D\) (spatiotemporal continuity),
\(C\) and \(D\) are brains of the same kind (e.g., human brains)
any change from \(C\) to \(D\) is gradual in nature

Problems:

No spatiotemporal continuity: brain replacement, psy. swap.
No same kind: artificial brain replacement
Fission problems: even when we meet all the conditions, we get a contradiction in fission cases.