Diachronic Personal Identity

Author

Affiliation

David W. Agler

The Pennsylvania State University

Abstract

Slides for Metaphysics.

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 \(C\).

Jane and Tek are similar (relative to their appearance)
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

Jane and Tek are twins, but they are not the same person.
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

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

Are the two Go stones qualitatively similar?
Are the two Go stones qualitatively identical?
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

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)

I’m chewing gum. The flavor is gone. It is same gum post-flavor-less
I have a Go stone and scratch it with my nail. It is the one and the same piece.
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

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