The “body swapping” thought experiment has traditionally been a principal argument in the arsenal for the psychological theory of personal identity (PTPI): The theory that some psychological relations (such as beliefs, desires, memories, or rational thought) are necessary for a person to be identical with himself. This thought experiment originates in John Locke’s paper, “Of Identity and Diversity,” in which he imagines the following scenario: Suppose a prince and a cobbler swap souls at a time t, so that the person who inhabits the body of the prince after t has the psychological attributes of the person who inhabited the body of the cobbler before t, and the person in the body of the cobbler after t has the psychological attributes of the person who was in the body of the prince before t. Locke concludes from this experiment that the criteria for the persistence of persons cannot be the same as the criteria for the persistence of matter, for although the physical attributes of the prince do not change after t, we no longer have the intuition that this same entity is the prince. In fact, we think that the cobbler is now in the prince’s body, and the prince now in the cobbler’s body. [1] Philosopher Bernard Williams argues that this thought experiment does not appropriately support PTPI. In his paper, “The Self and the Future,” Williams first presents a variation of the body swapping thought experiment, and then presents a second thought experiment which, while having the original characteristics of first experiment, results in a contrary conclusion: That psychological relations are not necessary for personal identity. [2] I argue that Williams’ second thought experiment is insufficient to produce this conclusion, given that it begs the question by rejecting a crucial assumption of PTPI: That the terms “me” and “this specific set of psychological attributes” are rigid designators which form an identity statement.

In his version of the body swapping thought experiment, Williams imagines that there is some process which two persons, A and B, will undergo to transfer the psychological attributes of A into B’s body, and transfer the psychological attributes of B into A’s body. We may call the resulting person with B’s body and A’s psychological attributes B-body-person, and the resulting person with A’s body and B’s psychological attributes A-body-person. Like the original Lockean account, it seems intuitively correct to say that A and B have switched bodies, so that B-body-person is really A, and A-body-person is really B. To make our intuitions clear, Williams opts to continue the experiment. Before this body swapping process occurs, the experimenter presents the following proposition: Of the two resulting persons (A-body-person and B-body-person), one will be given $100,000, while the other will be tortured. The experimenter then asks both A and B before the experiment occurs which treatment should be dealt out to which resulting person. Suppose that A and B choose out of purely selfish grounds (each wanting personally to receive the $100,000 and the other to be tortured), and, in this case, B chooses that A-body-person receive the $100,000 and B-body-person is tortured, while A chooses the converse.[3] The experimenter announces the day before the experiment that he will abide by B’s request, and, after the process occurs, he rewards A-body-person with the $100,000 and tortures B-body-person. In this case, B-body-person will adamantly complain (since he has A’s memories) of being tortured, since this was not the outcome that he requested, while A-body-person will express satisfaction (because he has B’s memories) since this was exactly what he requested. As Williams notes, the more details we provide to the thought experiment, the more we cannot escape from the intuition that A and B have truly swapped bodies: That A-body-person really is person B, and that B-body-person really is person A. Hence, it seems that we have evidence to affirm PTPI.

In the second half of the article, Williams presents a second thought experiment with the same characteristics as the first, the only notable difference being that, the second one will produce the contrary conclusion that PTPI is false. Suppose that a mad scientist tells me that I will be tortured tomorrow. Given that I, like the majority of people, desire to avoid pain, I will be fearfully anticipating the next day when this cruel process will be carried out upon me. However, the mad scientist adds another condition: That, before the torture is carried out, he will administer a powerful amnesiac, which will cause me to forget ever being told that I would be tortured. This surely will not yield me any comfort, for I can easily imagine being unexpectedly tortured because I had forgotten the prediction of the torture, and this is still a situation that I deeply fear. The mad scientist further adds that, not only will I not remember the prediction of the torture when the torture is administered, but I will also receive an entire and different set of memories and psychological traits from another person. According to Williams, this second condition still will not alleviate any fears,

“For I can at least conceive the possibility, if not the concrete reality, of going completely mad, and thinking perhaps that I am George IV or somebody; and being told that would have no tendency to reduce the terror of being told authoritatively that I was going to be tortured, but would merely compound the horror.”[4]

If Williams’ analysis of this second thought experiment is correct, then it serves a deadly blow to the argument that the traditional body swapping thought experiment provides support for PTPI. In the first thought experiment, we have the intuition that A remains the same person because his psychological attributes are preserved, despite switching bodies with B (the same goes for B as well). However, in the second thought experiment (which is essentially similar to the second), we have the intuition that I remain the same person despite the fact that my psychological attributes completely change.

The major mistake in Williams’ second thought experiment is that it begs the question against the proponent PTPI. For a thought experiment to be counted as successful counterevidence against a theory x, the thought experiment must not rely upon crucial assumptions which are rejected by x. To give an example of an appropriate thought experiment, take Ned Block’s famous case against functionalism: The Chinese Nation. In this thought experiment, Block imagines that every citizen in the nation of China implements the functions of the neurons in the brain. Every citizen is given a list of phone numbers and is instructed to call another citizen depending upon whom he receives a call from. If citizen a calls citizen b, b’s call list instructs him to call citizen c. But if citizen d calls b, b’s call list instructs him to call citizen e, and so forth. Due to the complexity of the call list, no actually phone message need be exchanged; the specific pattern of calls carries a certain meaning. These call-lists would implement the same patterns of activation that occur between neurons in the human brain. Take the state of pain, for example. China will function in an identical way to the brain when the brain produces sensations of pain, with citizens passing signals (or calls) to other citizens in the corresponding way that neurons pass signals to other neurons. However, while it makes sense to say that the brain is in pain, it makes no sense to say that China is in pain.[5]

This thought experiment counts as successful counterevidence against the theory of functionalism since it relies upon assumptions that the functionalist accepts, but produces an end result that the functionalist is unwilling to countenance (namely, that the nation of China can experience pain if given the appropriate functional system).

Following this line of reasoning, Williams’ second thought experiment only counts as successful counterevidence against PTPI if the thought experiment does not rely upon assumptions that PTPI rejects. To see how the thought experiment fails this condition, it will be necessary to employ a technical philosophical concept from modal logic and the philosophy of language: The concept of rigid designators.

A rigid designator is a term that refers to the same object in all possible worlds in which that object exists, and never designates anything else but that object. To give a classic example, the term “Mark Twain” is a rigid designator since it refers to the same object in every possible world where it exists (we may think of the object as instantiating the collection of properties which are necessary for Mark Twain to exist). There is no world in which the entity “Mark Twain” exists and yet he is not Mark Twain.[6] This is equivalent to saying that there is no world in which Mark Twain exists and yet does not have all of the necessary properties that make him Mark Twain. However, the term, “the author of Tom Sawyer,” is not a rigid designator, since it does not refer to the same object in every possible world in which it exists. The property of being the author of Tom Sawyer is by no means a necessary property of Mark Twain: There is a possible world in which Mark Twain exists, but he is not the author of Tom Sawyer. In fact, metaphysically speaking, for any person x, there is a possible world in which x could be ascribed the property of being the author of Tom Sawyer.[7]

Furthermore, if two rigid designators form an identity statement, and if the identity statement is true, then it follows that the identity statement is necessary, or true in all possible worlds. For example, the term “Samuel Clemens,” being a proper name, is a rigid designator for the same reason that the term “Mark Twain” is a rigid designator. So the statement, “Samuel Clemens is identical with Mark Twain,” if true, must be necessarily true, since the terms both refer to the same object in all possible worlds, and it is not possible that the terms could refer to any other object.

According to PTPI, the two terms, “me,” and, “this specific set of psychological attributes,” are rigid designators and form the following necessary identity statement: The person that “me” refers to is identical with the person that “this specific set of psychological attributes” refers to. The theory holds that I cannot be the same person if my specific set of psychological attributes changes.

In Williams’ second thought, the first step of the mad scientist’s process is applied to me: The amnesiac is administered, and I forget the prediction that I will be tortured tomorrow. The second step is also applied to me: My own psychological attributes are replaced with the psychological attributes of another person so that I no longer have my own psychological attributes but someone else’s. However, this is exactly the step that PTPI rejects, namely, that a person can remain identical with himself if he loses his psychological attributes. By imagining the exchange of psychological attributes as occurring to a persisting “me” who remains identical throughout the process, the argument presupposes the falsity of PTPI. At this point, the proponent of PTPI may simply reject this step in the thought experiment. There is no such thing as a separation of “me” and “this specific set of psychological attributes” since both terms are rigid designators, and the first is identical to the second. According to PTPI, it makes as much sense to say that a person’s psychological attributes can be exchanged and still remain the same person as it does to say that the Eiffel Tower could be melted down for scrap metal and yet still exist as the Eiffel Tower. When my psychological attributes are exchanged, I no longer exist. Rather, according to PTPI, the mad scientist’s process has created a new person.

To take stock, Williams argues that body swapping thought experiments do not demonstrate the truth of PTPI, since similar thought experiments may be contrived in which a person remains identical with himself, but whose psychological attributes are exchanged for different ones, thus showing that PTPI is false. For any thought experiment to serve as appropriate counterevidence against a theory, the thought experiment must not rely upon crucial assumptions that are rejected by the theory. PTPI asserts that the two terms, “me,” and, “this specific set of psychological attributes,” are rigid designators and form an identity statement where “me” is identical to “this specific set of psychological attributes.” Williams’ second thought experiment, however, presupposes that this assumption is false in order to arrive at a conclusion contrary his first thought experiment, namely, that PTPI is false, thus begging the question against PTPI and failing to demonstrate a case in which PTPI is actually false. The upshot is that Williams’ second thought experiment fails to provide an appropriate modification of body swapping thought experiments that produces the conclusion that PTPI is false.

[1] Locke, John. “Of Identity and Diversity,” An Essay Concerning Human Understanding. Prometheus Books, 1995.

[2] Williams, Bernard. “The Self and the Future,” Philosophical Review 79, no. 2 (1970): 161-180.

[3] Williams adds as a side note, “[T]his might indicate that they thought that ‘changing bodies’ was indeed a good description of the outcome” (163).

[4] Williams, 168.

[5] Block, Ned. “Troubles with functionalism,” Minnesota Studies in the Philosophy of Science 9 (1978): 261-325.

[6] Note: There is a distinction between the name, “Mark Twain,” and the entity, “Mark Twain.” Mark Twain could certainly have been called “Benjamin Franklin,” but he could not have actually been Benjamin Franklin (that is, he could not have had the properties that are necessary for Benjamin Franklin to exist), for then he would not be Mark Twain.

[7] That is, there is a possible world in which x actually wrote Tom Sawyer.