The year is 3006. You want to go to Jupiter. The trip takes three weeks by spaceship, but if you use a teleporter you'll be there almost instantly. The problem is you've never teleported before and you're a bit nervous. "Don't be a pussy", your friend says, "I've done it loads of times, it's fine." The apathetic assistant at the Teleportation Centre says you won't feel a thing. You'll press a button and the computer will copy the location of every cell in your body, down to the last atom. Your body will then be disintegrated, and an exact copy will be made at the Teleportation Centre on Jupiter. You will wake up on Jupiter, exactly as you were the moment you pressed the button. Despite your trepidation, you don't want to chicken out and look like a wimp, so you go into the teleporter and press the button. You wake up on Jupiter. You check your face in the mirror and it's just as it was before. Everything is present and correct, even that tiny scar on your knuckle, the one you got from a bat when you were eight. With your fear of teletransportation conquered, you start using it regularly.
One day, however, something goes wrong. On a routine journey from Pluto to Mars, you press the button but your body is not destroyed. An assistant tells you that he's sorry but there was a malfunction with the computer, and it failed to disintegrate your body. "Ain't no thang", you say nonchalantly, "just try it again". The assistant says he can't do that because the copying went ahead without flaw. He turns on the communicator beside him and, lo and behold, you see yourself standing there on Mars. The question is - Which one is the 'real' you? Instinctively you say the original, the one on Pluto, is the real one and the other version is just a copy. The problem with this answer is that before the machine malfunctioned, you were teleporting all over the place and you were happy to say that the copy was still 'you'. Does a copy become the real thing if the original ceases to exist?
The above scenario was devised by Derek Parfit in his book 'Reasons and Persons', and it poses some very interesting questions. What does it take for you to be you? This is the problem of personal identity. There are two opposing schools of thought regarding what is believed to be necessary for personal identity. The first is the Physical Criterion, whose proponents maintain that the copy is only the real you if it has enough of your brain to be a living person. From this point of view, personal identity lies with the brain and only when that brain is present and operational is identity guaranteed. The alternative view to this is the Psychological Criterion, which relates to psychological continuity and connectedness. Advocates of the Psychological Criterion, such as Parfit, believe that personal identity consists in the copy being psychologically continuous with the original, that they share the same personality and memories.
So which of the two is the better solution to the teleportation problem? Instinctively most people would say the original is the real you, and the Physical continuity theory supports this viewpoint. Psychological continuity theorists, on the other hand, would argue that both versions of you have an equal claim to being the real you. The copy is psychologically continuous with the original and that is all that is needed. Which of these two opinions is more convincing? I would have to go with the Physical criterion, because it enables someone to retain their numerical identity. There are two types of identity - Numerical and Qualitative. Numerical identity means countability as one, while Qualitative identity relates to the properties and qualities of something. Two white billiard balls may be qualitatively identical - they share the same size, colour and weight, but they could never be numerically identical because they are two numerically distinct objects. When we worry about dying, it's our numerical identity we're worried about, not our qualitative identity. We fear non-existence, not the alteration of some our properties and the Physical criterion protects this. The conclusion I make from this is that when you teleport - you die. You cease to exist and your numerical identity is lost. A copy is made but it isn't you, and never could be.
That is why I would never use a teleporter.