Diachronic Personal Identity

David W. Agler

The Pennsylvania State University

Diachronic Personal Identity

Diachronic Personal Identity

The question of diachronic personal identity concerns specifying the conditions under which a person \(A\) at \(t_1\) is one and the same person as person \(B\) at time \(t_2\).

Qualitative similarity

What does it mean to say two objects are the same?

Qualitative and Numerical Identity

Qualitative similarity

“Same” = qualitatively similar

Qualitative similarity

Objects \(A\) and \(B\) are qualitatively similar iff they share most of the same intrinsic properties \(F\) relative to some comparative class \(C\).

  1. Jane and Tek are similar (relative to their appearance)
  2. Kobe and Mike are similar (relative to their playing styles)

Qualitative similiarity and identity

  • When two objects \(A\) and \(B\) are qualitatively similar, we do not mean that \(A=B\).
  • \(A\) and \(B\) are not one and the same object.

Examples

  1. Jane and Tek are twins, but they are not the same person.
  2. Kobe and Mike are both basketball players, but they are different.

Qualitative identity

“Same” = qualitatively identical

Qualitative identity

Objects \(A\) and \(B\) are qualitatively identical iff they share all of the same intrinsic properties \(F\).

More precisely, objects \(x\) and \(y\) are qualitatively identical iff for any property \(F\), \(x\) is \(F\) iff \(y\) is \(F\).

Intrinsic properties

When thinking about qualitative identity, generally people are talking about the object’s “intrinsic properties”.

Intrinsic property

A property \(F\) of an object \(O\) is an intrinsic property iff \(O\) has that property in virtue of what \(O\) is and not in virtue of its relation to something else.

Intrinsic properties: Examples

Intrinsic properties (depend on the structure)

  • Mass
  • boiling point
  • ductility (stretchiness)
  • hardness

Extrinsic properties

  • comparative height (taller than, shorter than)
  • location: to the left of \(x\)
  • weight (depends upon gravitational field)

Qualitative identity examples

Example 1 - Two Cubes

  • Imagine two cubes \(A\) and \(B\) that are perfectly alike in everyway.
  • These two cubes would be qualitatively identical

Example 2 - Metaphysics Teacher

  • “The person who teaches Metaphysics”. Call it \(A\)
  • “David Agler”. Call it \(B\)
  • Here \(A\) is qualitatively identical to \(B\)

Numerical identity

“Same” = numerically identical

Numerical identity

Object \(A\) is numerically identical to \(B\) iff \(A=B\), viz., they are one and the same object.

  • “David Agler” and “David W. Agler” are one and the same person. They are numerically identical.
  • At time \(t_1\), take two cubes \(C_1\) in New York and \(C_2\) in Chicago. They are not one and the same object: \(C_1\neq C_2\).

Numerical identity - Counting

Counting reveals beliefs about numerical identity.

  • When we count, we count individual distinct things
  • When we assign an object a place in the count (1 apple, 2 apples) we treat it as a single, distinct entity.
  • If \(A=B\), then there is one thing, described in two ways.
  • If \(A\neq B\), then there is two things

Numerical identity: Ignorance

We sometimes don’t know that the objects picked out by our descriptions of objects are numerically identical.

  • “The morning star” - the bright light I see in the morning
  • “The evening star” - the bright light I see in the night.

“The morning star” = “The evening star” = Venus

Numerical identity: Dennis Rader

  • My neighbor is Dennis Rader. He works as a compliance officer with me.
  • I just saw a report on the BTK Killer

Dennis Rader = BTK Killer

Qual identity entails num. identity?

It is debatable whether qualitative identity entails numerical identity.

Test case: Imagine two cubes \(A\) and \(B\) that have the same intrinsic properties but are in different locations. Are \(A\) and \(B\) one and the same object?

Space and identity

People think there are two cubes \(A\neq B\). Even though location is not an intrinsic property of a thing, it plays a role in distinguishing objects.

  • If \(A\) and \(B\) are in different locations, then they are different objects \(A\neq B\)

In contrast, people think there can only be one object wholly occupying a space.

  • If \(A\) and \(B\) are in the same location, then \(A=B\)

Num. identity entails qual identity?

People debate whether numerical identity entails qualitative identity:

Take John at time \(t_1\) and then 1s later at \(t_2\). Some of John’s properties have changed. So the two “Johns” are not qualitatively identical, but people will say there is one John (numerical identity).

  • If you think there is one John, then you deny that num. identity entails qualitative identity.
  • One and the same object can survive change.

Exercise

Exercise

Consider the Go stones given to you. Answer the following:

  1. Are the two Go stones qualitatively similar?
  2. Are the two Go stones qualitatively identical?
  3. Are the Go stones numerically identical?

Diachronic numerical identity

Diachronic numerical identity has to do with an object being numerically identical through time (or more generally, at different times)

Diachronic numerical identity

Object \(A\) at \(t_1\) is numerically identical to \(B\) at \(t_2\) (where \(t_1\neq t_2\)) iff \(A=B\), viz., they are one and the same object.

Question of diachronic numerical identity

Question of diachronic numerical identity

The qustion of diachronic numerical identity is how does an object \(O\) survive change? Some changes destroy the object, others preserve its identity, under what conditions is an object \(O\) at \(t_1\) one and the same object as \(O\) at \(t_2\)?

Examples (dia numerical identity)

  1. I’m chewing gum. The flavor is gone. It is same gum post-flavor-less
  2. I have a Go stone and scratch it with my nail. It is the one and the same piece.
  3. Microscopic proteins inside my skeletal muscles replace themselves every 2-3 months, skeletal muscle cells are slowly replace every 15 years, my skeleton is replaced every 10 years. I survive all of this change!

Exercise

Exercise

Create an example of an object \(A\) at \(t_2\) that is qualitatively different than \(B\) at \(t_1\) but it is one and the same object. That is, create an example where an object survives change

Diachronic personal identity

The question of diachronic numerical personal identity is just the question of diachronic numerical identity applied to persons.

  1. Narrow version (most popular)
  2. Broad version (less popular)

Narrow version

Narrow version

Under what conditions is it the case that person \(A\) at \(t_1\) is one and the same person as \(B\) at \(t_2\) (where \(t_1\neq t_2\))?

  • You have two persons at two different times.
  • When are they the same person?
  • Assumption: you only can exist as a person

Broad version

Broad version

Let \(A\) be a person at \(t_1\) and \(B\) be an object at \(t_2\) (where \(t_1\neq t_2\)). Under what conditions is it the case that \(A=B\)

Exercise

It is possible that you could continue to exist as a non-person?

  • If you were modified slowly over time into a non-personal animal, would you be you?
  • If a human fetus is not a person, then were you ever a fetus?

Why Care?

Why care about diachronic identity?

The question of diachronic personal identity is:

  1. What sorts of changes can a person survive?
  2. If \(A\) is a person at t1 and \(B\) a person at t2, then under what conditions is it that \(A=B\)?

But why care?

Reason 1: Survival

  • Risky medical operation
  • Large change to personality
  • Large modification to body, e.g., brain implant
  • Spatial transportation
  • Mind uploading: upload consciousness to the cloud

Reason 2: Self-Understanding

My beliefs about my history inform how I understand myself today and my place in the world

  • Was I a fetus?
  • Am I the same person as I was before became an adult?
  • Psychological attitudes: No guilt about things “I” didn’t do.

Reason 3: Punishments

We only punish the person \(A\) that did the bad act.

  • We don’t punish Tek if Liz did the bad deed.
  • If Tek at \(t_2\) is not the same person at \(B\) at \(t_1\), then it would be wrong to punish Tek for what \(B\) did.

Reason 3: Rewards

We only reward the person \(A\) that did the good (meritorious) act.

  • Mistake to give Tek praise if Liz did the good deed.
  • If Tek at \(t_2\) is not the same person at \(B\) at \(t_1\), then it would be mistaken to reward Tek for what \(B\) did.

Reason 4: Sacrifices

We often forego immediate utility for greater future utility. This self-interest is only rational if the person in the future is you.

  • Put money in retirement instead of spend it all.
  • Avoid smoking to avoid future lung-cancer
  • Study today to get A tomorrow.

Continued

People consign their future selves to pay the costs of activities done by their present selves.

  • Procrastination
  • Bad health decisions
  • Poor financial habits

Why?

Exercise

When our present self makes a decision that would significantly burden our future self, it seems like we are viewing our future self as someone else?

  • Are we just thinking that future me \(\neq\) “not me”?

Neuroimaging results

vMPFC (brain) is active when people think of themselves

  1. When rating how much they would enjoy an activity \(A\), vMPFC response was identical to how much they think another person would enjoy \(A\).1
  2. People with less vMPFC response when considering actions that impact their future self tend to prefer small immediate rewards as opposed to large future rewards.

Exercise

Exercise

  1. Suppose you could have a conversation with your future you. What sorts of things might they ask you to currently change?
  2. If we do tend not to view our future self \(B\) as one and the same person as our present self \(A\) — that is, we tend to feel \(A\neq B\) —, what sort of practices can we engage in to better feel / believe / be reminded that \(A=B\)?