Leibniz famously offers this thought experiment:
Supposing that an individual were to instantly become King of China, but on the condition of forgetting what he has been, as if he was completely born again—isn’t that practically, with regard to perceivable effects, as if he were to be annihilated and a King of China were to be created in the same moment in his place? This the individual has no reason to desire. (Gerhardt IV, p. 460)
The context is that Leibniz isn’t doing metaphysics here, but supporting an ethical point that memory is needed for one to be a fit subject for reward and punishment and a theological point that eternal life requires more than mere eternal existence without the psychological features of human life. Nonetheless, some have thought that thought experiments like Leibniz’s offer support for memory theories of personal identity. I will argue that tweaking Leibniz’s thought experiment in two ways shows that this employment would be mistaken. In fact, I think the second tweak will offer an argument against memory theories.
Tweak 1: Memory theories of personal identity require a chain of memories, but not a chain of personally important memories. So all we need to ensure identity of the earlier individual with the later King of China is that the King of China remembers something really minor from the hour before the transformation, say seeing a fly buzzing around. Allowing the memory of a fly to survive the enthronement does not affect the intuition that the process is one that “the individual has no reason to desire.” The loss of personally important memories—especially of interpersonal relationships—is too high a price for the alleged benefit of ruling a great nation. Hence the intuition is not about personal identity, but—as Leibniz himself thinks—about prudential connections in a person’s life. Nor should we modify memory theories of personal identity to require the memories to be personally important, since that would make personal identity too fragile.
Tweak 2: First, suppose that in addition to the individual’s memories being wiped, the individual gets a new set of memories implanted, copied from some other living person. That so far does not affect the intuition that the process is one that one has “no reason to desire.” Second, add that the other living person happens to be one’s exact duplicate from Duplicate Earth. On memory theories of personal identity, one still perishes—the memories aren’t one’s own, even if they are exactly like one’s own. But a good chunk of the force of the thought experiment evaporates. It is, admittedly, an important thing that one’s apparent memories be real memories, and when they are taken from one’s exact duplicate, they are not. If one’s apparent memories are from one’s duplicate, then one isn’t remembering one’s friends and family, but instead is having quasi-memories of the duplicate’s friends and family, who happen to be exactly like one’s own. That is a real loss objectively speaking. But it is a much lesser loss than if one’s memories are simply wiped or replaced by those of a non-duplicate.
Note further that in the case where one’s memories are replaced by those of a duplicate, if enough benefits are thrown into the King of China scenario, the whole thing might actually become positively worthwhile. Suppose you are a lonely individual without significant personal relationships, but as King of China you would have a fuller and more interpersonally fulfilling life, despite the inevitable presence of flatterers and the mind-numbing work of ruling a vast empire. Or suppose creditors are hounding you night and day. Or you have a disease that can only be cured with the resources of a vast empire. When we note this, we see that the modified thought experiment provides evidence against the memory theory. For on the memory theory, it makes no difference to one’s identity whether the memories will come from a duplicate or not, as long as they don’t come from oneself, and what benefits the King of China will receive is largely prudentially irrelevant.
Objection 1: If the King of China gets memories from your duplicate, then the King of China will have your values and will promote your goals with all of the power of an empire. That could be prudentially worth it and provides some noise for the tweaked thought experiment.
Response: We can control for this noise. Distinguish your goals into two classes: those where your existence is essential to the goal and those where your existence is at most incidental to the goal. We can now suppose that you are a selfish individual who has no goals of the second type. Or we can suppose that all your goals of the second type are such that you think that being King of China will not actually help with them. (Perhaps world peace is a goal of yours, but like Tolstoy you think individuals, including emperors, are irrelevant to such goals.)
Objection 2: If you know that the duplicate has the exact same memories as you do, then copying memories from the duplicate at your behest maintains a counterfactual connection between the final memory state and your pre-transformation memories. If the latter were different from what they are, you wouldn’t have agreed to the copying.
Response: There is nothing in Leibniz’s thought experiment about your consent. We can suppose this just happens to you. And that it is a complete coincidence that the subject from whom memories are taken and put into you is your duplicate.
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