1. Chapter 4 The Problem of Personal Identity

2. The Problem of Change
How can something change and yet remain the same thing?
If something changes, it’s different. And if it’s different, it’s no longer the same.

3. Qualitative vs. Numerical Identity
Two objects are qualitatively identical if and only if they share the same properties (qualities); two cue balls from the same manufacturer, for example.
Two objects are numerically identical if and only if they are one and the same; Mark Twain and Samuel Clemens, for example.

4. Accidental vs. Essential Properties
An accidental property is one that something can lose without ceasing to exist. Your hair being a certain length, for example, is an accidental property of you.
An essential property is one that something cannot lose without ceasing to exist. Your being a person, for example, is an essential property of you.

5. Thought Probe: A Different Person
Kathleen Soliah certainly seems to be a qualitatively very different person than she was in the 1970s.
Is she numerically different?
Is she different enough that she should get a reduced sentence?

6. Section 4.1 We Are Such Stuff as Dreams are Made On
Self as Substance

7. Locke on Identity Conditions
Masses of matter like rocks or lumps of clay retain their identity as long as they retain the atoms out of which they are made.
Living things like plants or animals retain their identity as long as they retain their functional organization.

8. Thought Probe: Hobbes’s Ship of Theseus
Suppose that the planks in Theseus’s ship have been replaced one by one over the years until none of the original planks remained.
Suppose further that the original planks were saved and put back into their original order.
Which ship is identical to the original?

9. Persons
Persons are generally considered to be rational, self-conscious beings that have free will.
They have full moral standing, including the right to life.

10. Non-human persons
According to animalism, there can be no non-human persons.
But rational, self-conscious computers, aliens, and animals seem possible.
So being a human may not be a necessary condition for being a person.

11. Animalism
Animalism is the doctrine that identical persons are identical human animals.
According to animalism, once our bodies die, we cease to exist.
So our only hope for eternal life is some form of resurrection.

12. Humans Who are Not Persons
Those who have fallen into a permanent vegetative state are no longer capable of being rational or self-conscious.
Even though they have a living, human body there is reason to think that they are no longer persons.
So being a human may not be a sufficient condition for being a person.

13. Body Switches
According to animalism, it’s impossible for one person to inhabit two different bodies.
But body switches as portrayed in the movies Big, Heaven Can Wait, All of Me, Freaky Friday, etc., seem possible.
So having the same body may not be a necessary condition for being the same person.

14. Thought Experiment: Locke’s Tale of the Prince and the Cobbler
“For should the Soul of a Prince, carrying with it the consciousness of the Prince’s past life, enter and inform the Body of a Cobbler…every one sees, he would be the same Person with the Prince.”
If such body switches are possible, animalism is false.

15. Thought Experiment: The Transplant Case
Imagine that an ingenious surgeon removes your cerebrum and implants it into another head.
Are you the biologically living but empty-headed human being that has inherited your vegetative functions?
Or are you the person who ends up with your cerebrum and your memories?
Or are you dead?

16. Thought Experiment: Unger’s Great Pain
Imagine you’re about to undergo a brain transplant.
Unger asks whether you would choose to have yourself suffer considerable pain before the transplant to prevent the person with your brain from suffering even greater pain afterwards.
If so, animalism is false.

17. Siamese Twins Some Siamese twins have two heads but one body.
Is this a counterexample to animalism?
Can animalism be modified to account for this counterexample?

18. Multiple Persons, One Body
According to animalism, it’s impossible for one body to contain two persons.
But in the case of multiple personality disorder, split-brain patients, and Siamese twins, it seems as if there are two persons in one body.
So having the same body may not be sufficient for being the same person.

19. Soul Theory
According the soul theory, identical persons are those with identical souls.
As long as your soul exists, you exist.
But what are the identity conditions for souls? What is it for one soul to be identical to another soul?

20. Souls and Thoughts
Souls are thinking substances. They are not thoughts; they are things that think.
The relation between a soul and its thoughts can be likened to the relation between a pincushion and its pins.
Just as a pincushion can have different pins in it at different times, the same soul can have different thoughts in it at different times.

21. Thought Experiment: The King of China
Suppose someone offered to make you the King of China on the condition that you lose all of your memories.
In such a case, it’s doubtful that you would be around to enjoy the wealth even though, presumably, your soul would be.
So having the same soul may not be a necessary condition for being the same person.

22. Thought Experiment: Nestor and Thersites
Suppose that all of your thoughts and memories were transferred to a soul that once housed those of Nestor or Thersites.
That would not make you identical to Nestor or Thersites.
So having the same soul may not be a necessary condition for being the same person.

23. Kant’s Soul Switch
“If…we postulate substances such that the one communicates to the other representations together with the consciousness of them, we can conceive a whole series of substances….The last substance would then be conscious of all the states of the previously changed substances as being its own states.”
So identity goes—not where your soul goes—but where your consciousness goes.