In An Essay Concerning Human Understanding, John Locke considers the notion of personal identity. He offers an account of what it means to be a person and what it means to say that a at time t1 is the same person at time t2. According to Locke, a person is a rational thinking being. Identity of a person over time then consists in continuity of the consciousness which accompanies the person’s thoughts. “As far back as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person” (quoted in Personal Identity. Ed. John Perry. University of California Press. 2008. page 39). Thomas Reid criticizes Locke’s views in his essay called Of Memory (also quoted in Personal Identity). Reid writes that the only thing Locke could have possibly meant when he wrote about extending consciousness backward is memory. Reid attacks the view that memory makes it the case that personal identity continues over time.
Reid believes that Locke has confused the evidence we have for personal identity with personal identity itself. He writes that it makes no sense to say that a person’s memory of having performed an action makes it the case that he or she is the person that performed the action. To speak like that is to say that “the testimony is the cause of the thing testified” (116). Since memory is supposed to be the testimony of personal identity, it cannot cause personal identity.
Reid illustrates his point with an example. The only way that we can tell if a given horse is the same horse as one we have seen earlier is by similarity of appearance. But we would not say that the identity of the horse just is similarity of appearance. It being the same horse explains why it looks the same, rather than it looking the same explaining that it is the same horse. Likewise, my being the same person makes it the case that I can remember my past actions, rather than my memory making it the case that I am the same person.
Locke wants to say that consciousness or memory is a necessary and sufficient condition for personal identity. Reid says that it cannot be a necessary condition (because it can be the case that someone performed an action without remembering that they performed it), though it is a sufficient condition. Locke, however, would not want to give up the idea that memory is a necessary condition for personal identity because he thinks it is the only principle that allows us to assert that one person cannot inhabit multiple bodies throughout history. He thinks it is impossible to show that an immaterial substance cannot inhabit multiple bodies. But if memory is a necessary condition for personal identity, Locke can say that even if an immaterial substance were to inhabit another body, it would not be the same person because the person in the new body cannot remember actions performed in another body.
Locke might respond to this by saying that he never meant to say that memory causes personal identity. He could then say that something else causes personal identity, but it is still the case that memory is a necessary condition for personal identity. We just always see continuity of personal identity coinciding with memory even though there is no causation. This will not work though because he would have to say that it just so happens that personal identity is caused only when memory is present. This would be quite an odd situation and he would then have to give a story about why this is the case.
Or Locke might say that it does not make sense to talk about causation of personal identity at all. He could say that when we talk about personal identity, we are not talking about some thing or organization of things physically existing in the world. Personal identity is simply something we ascribe to a man and as such it is not caused at all. Locke’s theory can be understood as an attempt to bring forth a criterion that describes when we would attribute personal identity. He often appeals to our intuition by asking whether in a given situation we would call someone the same person or not. In this case, the ideas of testimony and thing testified don’t come up, so Reid’s objection is groundless.
I think this defense by Locke is on the right track. Personal identity is only ever an epistemological question, never a metaphysical one. At best, the discussion of personal identity is about the conditions under which we would judge personal identity to continue. So whatever criteria is offered needs to correctly predict the instances in which we have a clear intuition about whether personal identity continues in a specific situation. It also needs to give a clear prediction for cases in which our intuition is not so clear. According to the intuition of most people, a personhood continues through drunken and sober states. This intuition is useful when deciding that a person be punished for what they do while drunk. In the case of an accident that causes amnesia or severe mental damage, our intuition is not so clear. A prediction for this case would be useful in deciding, for example, whether the person is responsible for upholding a contract. The predictions based on the memory theory seem to coincide with our intuition in most cases, but it will need some fine tuning to get it where we need it to be.
However, even with this fine tuning, memory theory can only predict in cases where we know whether an individual has memory. Memory is currently inaccessible from third-person positions and it is not clear whether memory can ever be accessible that way. Memory is sometimes inaccessible even from a first-person position. Ideally, we want a theory that accurately predicts personal identity based on something we know to be accessible from a third-person perspective.