Boy, did Theo had problems with that car he bought at Honest Joe’s. It started off with little things. A doorlock needed replacing. And some fiddly bits in the rear suspension fell off. The usual. Then bigger stuff started to go wrong. First the clutch, then the gearbox… Finally the whole transmission. The tale of Theo’s car, or more usually the Ship of Theseus, is one of the many puzzles used by philosophers to test intuitions about the identity of things or persons over time. It seems our intuitions in these areas are often strong but conflicting. The story of Theseus’ Ship was told by the English philosopher Thomas Hobbs who then elaborated further. To pick up on Theo’s version: Honest Joe’s didn’t live up to his name. Most of the bits he replaced in Theo’s car were working fine. And he mended any that weren’t. He had saved the old parts and was fitting them together. After two years, he had assembled an exact copy of Theo’s car. He thought it was a copy, but maybe it was Theo’s car.

Which is the original? The car Theo has, now entirely build of new parts, or Joe’s version build entirely of the original parts?

The identity of the car over time is not nearly as neat and tidy as we might wish. It isn’t just a problem with cars and ships, people change enormously over a lifetime. Physically and psychologically, there may be very little in common between a two year old toddler and a 90 year old who has taken his place 88 years later. So are they the same person? It they are, what makes them so? This is the problem of personal identity which has kept philosophers busy for hundreds of years. So what just are the necessary and sufficient conditions for a person to be the same person at one time and a later time?

Animals and brain transplants
The common sense view is probably that personal identity is a matter of biology. I am now who I was in the past because I am the same living organism, the same human animal. I am linked to a particular body which is a single and continuous organic entity. But imagine for a moment a brain transplant. An operation we can envision to be in reach of future technology. In which your brain is transferred into my body. Our intuition is surely that you then have a new body. Not that my body has a new brain. This consideration has lead some philosophers to retreat from body to brain. To claim identity is linked not to the whole body, but to the brain.

Micaela Lattanzio’s project ‘Fragmenta’. Found here….

This move fits our intuition regarding the brain transplant case, but still does not quite do the job. Our concern is what we suppose emanates from the brain, not with the physical organ itself. While we may be uncertain how brain activity gives rise to consciousness or mental activity, few doubt the brain actually underlies that activity. In considering what makes me me, it is the software of experiences, memories, beliefs, et cetera, that concerns me, not the particular lump of grey matter. My sense of being me would be not much shaken if the total sum of those experiences were copied onto a synthetic brain. Or indeed of someone else’s brain could be reconfigured to hold my memories, beliefs, et cetera. I am my mind. I go where my mind goes. Based on this view, my identity is not linked to my physical body, including my brain, at all.

Psychological continuity
Taking the psychological approach to the question of personal identity, rather than a biological or physical one, let’s suppose that each part of my psychological history is joined to earlier parts by strands of enduring memories, beliefs, et cetera. Not all, and perhaps none, of these need extend from start to finish. Provided there is a single overlapping web of these elements. Then it remains my history. I remain me. The idea of psychological continuity as the main criterion of personal identity comes from John Locke. It is the dominant theory among contemporary philosophers, but is not without problems of its own.

Imagine for instance, a Star Trek style teleportation system. Supposed this records your physical composition down to the last atom and then transfers this data to some remote location. Let’s say from London, earth, to Moonbase 1. There your body is exactly replicated from new matter at the precise moment your body in London is annihilated. All is well, as long as your psychological self is also exactly copied. But now suppose the transporter failed to carry out the annihilation in London. Now there are two of you: one on earth and one on Moonbase 1. According to the continuity account, because the psychological continuity is preserved in both cases, they are both you. In this case we have little hesitation to saying that you are the individual in London while the one on the moon is a copy. But if this intuition is right, we seem to be forced back from the psychological to the biological animal account. It appears to matter that you are the old meat in London rather than the new meat on the moon.

Getting yourself straight
Such mixed intuition may come from asking the wrong questions or applying the wrong concepts in answering them. David Hume drew attention to the elusiveness of the self claiming that however hard you look in on yourself, you can only ever detect individual thoughts, memories, experiences. While it is natural to imagine a substantial self, that is the subject of these thoughts, he argues this is wrong. The self is no more than the point of view that makes sense of our thoughts and experiences. This idea of the self as a substantial thing, which we take to be our essence, causes confusion if we imagine our self undergoing brain transplants or being annihilated and reconstituted somewhere else. We assume our personal survival in such thought experiments somehow depends on finding a place for this self. But if we stop thinking in terms of this substantial self, things become clearer. Suppose for instance that the teleporter functions correctly in annihilating your body in London, but producing two copies on the moon. Asking who is you is now simply asking the wrong question. The outcome is that there are now two human beings, each starting off with exactly the same stream of thoughts, memories and experiences. They will now go their own way and their psychological histories from now on will diverge. You, essentially the fond of thoughts, experiences, et cetera, have survived into two new individuals, an interesting form of personal survival, but achieved at the cost of your personal identity.

Source: 50 philosophy ideas you really need to know (Ben Dupré)