On some views, what we are, or have as an essential part, is a soul: an immaterial persisting entity, which is indivisible, and whose continued existence must be all-or-nothing. Views about what we are, and how we might continue to exist, can be placed, roughly, in three main groups. We can also ask what kind of entity we are, since entities of different kinds continue to exist in different ways. To answer such questions, we must know the criterion of personal identity over time, by which I mean: the relation between a person at one time, and a person at another time, which makes these one and the same person. Thus, when using the telephone, we might ask whether the person to whom we are speaking now is the same as the person to whom we spoke yesterday. In most cases, we use descriptions that refer to people at different times. In questions about numerical identity, we use two names or descriptions, and we ask whether these refer to the same person. That, we naturally assume, is what matters. And in my imagined case of Teletransportation, my Replica on Mars would be qualitatively identical to me but, on the sceptic’s view, he wouldn’t be me. I shall still be alive if there will be someone living who will be me. I may believe that, after my marriage, I shall be a different person. In our concern about our own futures, that is what we have in mind. But I shall be discussing our numerical identity.
That is the question involved, for example, in an identity crisis. When people discuss personal identity, they are often discussing what kind of person someone is, or wants to be. This numerically identical person is now qualitatively different. That is not a contradiction, since it means that this person’s character has changed. This claim involves both senses of identity, since it means that she, one and the same person, is not now the same person. Consider next a claim like, ‘Since her accident, she is no longer the same person’. If I paint one of these balls red, it will cease to be qualitatively identical with itself as it was but it will still be one and the same ball. But they are not numerically identical, or one and the same ball. Two black billiard balls may be qualitatively identical, or exactly similar.
To describe such disagreements, we can first distinguish two kinds of sameness. This disagreement is about personal identity. On their view, the person who wakes up would be a mere Replica of me. Others believe that, if I chose to be Teletransported, I would be making a terrible mistake. They regard Teletransportation as merely the fastest way of travelling. Of those who have thought about such cases, some believe that it would be I who would wake up on Mars. The person who wakes up on Mars seems to remember living my life up to the moment when I pressed the button, and is in every other way just like me. This information is sent by radio to Mars, where another machine makes, out of organic materials, a perfect copy of my body. When I press some button, a machine destroys my body, while recording the exact states of all my cells. 1 Here on Earth, I enter the Teletransporter.