Essential identity: Physical identity

In continuing the discussion of our intuitive concept of personal identity, I look at ways in which our physical attributes might be the source of our identity.

In some senses, identity is a physical fact. No other person exists or has existed on this planet who shares my genome. Well, actually this is just a statement of probability. It is after all possible that some other person with my precise genome has existed in the past or even exists concurrently with me in some other place. A doppleganger.

But we all know that genetic identity is not the same thing as true identity. Identical twins share a genome and yet we recognise that of course they are distinct individuals. We would not deliberately seek to punish twin A for the crimes of twin B. If we discovered that some mad scientist had managed to clone Hitler, most of us would probably not choose to punish the clone for the crimes of the original.

So what is it that distinguishes one twin from another?

Well, twins are not precisely identical after all. There are minor differences in physical appearance between even the most alike twins. But would we consider them to be the same person if we could find no such physical differences? Of course not. So physical uniqueness cannot account for identity.

But what if we could make a complete clone of a person, copying their minds along with every atom of their body? It's not clear according to our intuitions whether we should now regard the two copies as being the same person or not. I suspect most of us would consider the original person to be the inheritor of the original identity, while the new person would get a new identity. The clone would not be entitled to any of that persons' possessions, and possibly shouldn't even be allowed to drink (being legally a newborn!). I will further discuss the ramifications of perfect cloning in a later post.

Even if two twins are physically indistinguishable, they do at least occupy distinct places in space. They are each formed of distinct sets of atoms. Perhaps that specific collection of atoms is the criterion that establishes identity.

This doesn't really work either. The atoms that make up our bodies are for the most part continuously recycled and replaced with those in our environment, taken in from food, drink and air and excreted in a number of ways.  For example the skin you wear today is not the same skin you were wearing 35 days ago -- your epidermis has been completely replaced. A good thing too, or it would wear out the way your clothes do, soon becoming unfit for purpose.

This is an example of the Ship of Theseus paradox, which asks whether an object which has had all its parts replaced remains essentially the same object. The human body is just one among the many fascinating examples listed on that Wikipedia link (recommended reading!). I know I've replaced so many components of my computer since I bought it that it seems difficult to explain why it is the same computer.

This problem can have practical implications. It is well illustrated by the case of software licenses which permit the use of the software on one specific computer only. Sometimes over-zealous digital rights management measures even disable software if they detect that some key components of the computer have been replaced, mistaking the upgrade for an attempt to copy the software to an unauthorised machine.

But none of us truly identify with our bodies. Rather, we identify with our brains or minds. It's the software, not the hardware that defines us. I'll discuss this next.