Why should we care about personal identity?

Question

I just read a book about personal identity and it gave me insights in different theories about personal identity. But after all, I still don't get, why we should care about personal identity.

Being a philosophy student, I can usually grasp immediately the reason, why we engage in some philosophical projects: What is justice? What is language? What statements follow necessarily, which statements are a posteriori? What is science? etc.

But what's so interesting about personal identity? If I can't remember, whether that guy in the school-picture was me or not, no amount of philosophizing will help me. If the police wonders, whether that guy sitting in that chair was the bank-robber two hours ago, no philosophical theory will help the police to distinguish the real robber from an innocent person. So, the study of personal identity has no practical value. But does it have any theoretical value?

Eliran asked:

It seems that you are asking two different questions. (1) What is the importance of personal identity as a subject in philosophy? (2) Why should people care about their personal identity? Can you clarify which one you mean (or do you mean both)?

I'd like to hear an answer to both questions, if possible.

Answer 1

The question of personal identity falls under the general heading of metaphysics, and so one answer to your question(s) is: We study the question of personal identity for whatever reason we study metaphysics.

Below are 4 other reasons why someone should study the question of personal identity:

• The legal definition of what is a person and is not has implications for financial, fiscal and freedom of speech issues: "Corporations are not people" is a slogan of the left in the US which is used when criticizing the US Supreme Court's citizens united decision to allow corporations to spend as much as they wanted on political campaigns. The premise of Citizens United was that corporations and other organizations should be accorded the same freedom of expression rights that an individual person is. If someone is to use the "Corporations are not people" argument against Citizens United, they should be equipped with a suitable definition of what a person is, which wouldn't be complete without a clear definition of personal identity.
• The concept of "personhood" is also used in the abortion debate in the US: Some anti-abortion activists consider that a "person" is created the moment a sperm fertilizes an egg, and thus any abortion (and many forms of contraception as well) constitutes murder. They use this line of argument to try to convince states (notably the state of Mississippi) to ban abortion on the ground that it is tantamount to murder. Again, one can't wade into the debate of whether a fertilized egg constitutes a real person or not unless they have a working definition of personal identity.
• Given that human level AI are now a realistic technological possibility (See for example Kurzweil's How to Create a Mind), the question of what constitutes a person and what doesn't is again relevant to the debate. Here, not only the question of personhood in general, but the explicit concept of personal identity is more specifically relevant: If I upload a self aware digital version of myself to a computer, is it a different person given that it is an autonomous and self-aware AI construct? is it an extension of me and I should be held responsible for anything it does, in the same way that a programmer is responsible for viruses and malware that they design? Etc...?
• Perhaps the most interesting application of the question personal identity is the therapeutic one: Buddhists hold that recognizing that the self is an illusion is beneficial and can help lead one to enlightenment and liberation from pain and desire. This is the concept of Anatta. More recently, British philosopher Derek Parfit arrived at a similar "liberation" upon losing his belief in the concept of separate self: "... seemed imprisoned in myself. My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness. When I changed my view, the walls of the glass tunnel disappeared. I now live in the open air. There is still a difference between my life and the lives of other people. But the difference is less. Other people are closer. I am less concerned about the rest of my own life, and more concerned about the lives of others." [Reasons and Persons, p. 281]

Answer 2

Derek Parfit in his book on Reasons and Persons puts forward the "Impersonality Thesis". The central claim of the Impersonality Thesis is that all reference to persons is eliminable in favor of reference to bodies and experiences: shortly put, reality can be completely and impersonally described.

What Parfit has in mind when he say this is the fact that a person's identity over time consists in holding certain, more particular facts. These facts can be described without either presupposing the identity of a person, or claiming explicitly that the experiences which a person has are had by "this particular person", or even explicitly claiming that "this particular person exists".

Or, to put it in another way, although we can assume that persons exist, we could still manage to give a complete description of the world, without claiming that persons exist. In Parfit's own words:

"Even when we have no answer to a question about personal identity, we can know everything about what happens". (Parfit 1984, 266)

We can talk about our selves through time without having to assume identity. Instead we can think in terms of continuity and degrees of connectivity between our selves.