Philosophy Now magazine has an interesting article on the problem of identity – how we have the impression that we are the same person, despite the fact that our personality, preferences and even cognitive abilities may change from moment to moment.
It’s a problem that was most famously tackled by 17th century philosopher John Locke but is still relevant for understanding the issues of identity and the self in contemporary cognitive science, as well as for informing complex judgements on free will and responsibility.
Suppose a man has committed a crime whilst drunk or undergoing temporary amnesia. Suppose also, that because of his mental state at the time of the offence, he genuinely cannot remotely remember a thing about it. Clearly on the evidence of witnesses ‚Äì and perhaps he was caught in the act ‚Äì it was his own body, the same man who now stands in the dock, who did it. But was it the same person? Should the present person be found guilty of the crime if the drunkenness or amnesia had so changed his psyche that, at the time, he ‘wasn’t his true self’? Can he rightly claim that at the time of the incident the occupant of his body was a different person altogether; or perhaps some fractured component of his own psyche that couldn‚Äôt rightly be described as ‚Äòhimself‚Äô?
Psychological continuity was, Locke claimed, the answer to the question. The accused, considered as a man, the physical being, is certainly guilty. His own hand struck the blow, his own voice had risen in anger. But if the person, the psychological being, cannot remember one atom of it, then he is not guilty.
But though Locke’s theory answered the question, it‚Äôs not certain that it solved the problem; for it raises a paradox that will try the wits of the jurists: the man in the dock may be guilty, but not the person in the man! And if the man is punished, he will experience the pain, but the wrong person will suffer it.
Link to article ‘A Question of Identity’ (via Thinking Meat).
Mind Hacks 
The other issue is not just about whether one is the same, but whether the previous person “dies”. For if we change from moment to moment, with experiences and memories changing moment to moment, what is the status of our previous selves? Are they dead? Are we dying and being reborn with every thought?
It seems ridiculous to think we are dying every nanosecond, since we feel alive and continuous at this moment, compared to 1 second ago, compared to yesterday. But it could be that you really have been alive for just that brief moment, just that our memories give us the impression we have been living for so many years. If you suspended my brain mid-comment, then gave it to a new person who was just activated a second ago to continue what I am writing, would he know any better?
I’ve also tried taking a page from linguistics and analysing the “self” in the way you would analyse a dialect continuum. In comparing yourself, to say, yourself 2 years ago, you can analyse the discrete differences in thoughts and memory as isoglosses.
I’m not a psychologist nor a law expert.. But my belief is that the “self” is guilty when it chooses to intoxicate its own body. Being under the influence of drugs or alcohol is a choice and the “self” is therefore accountable for the consequences .
I don’t know what effect amnesia has on the psyche, how character changes, and what effect this changed character has on the “self” that returns when the amnesia fades.
Some killers defend themselves in court by testifying they killed while asleep. As with intoxication and amnesia such things are very hard to prove or disprove.