"[Bernard] Williams suggests that loving a person is, basically, loving a particular body. But this kind of love, or lust, is at most extremely uncommon. What is more common is a purely physical or sexual obsession with a person’s body, an obsession that is not concerned with the psychology of this person. But this is not love of a particular body. As Quinton writes, in the case of such obsessions, ‘no particular human body is required, only one of a more or less precisely demarcated kind’. Suppose that I was physically obsessed with Mary Smith’s body. This obsession would transfer to Mary Smith’s Replica. This would be like a case in which the body with which I am obsessed is that of an identical twin. If this twin died, my obsession could be transferred to the body of the other twin. Ordinary love could not be so transferred. Such love is concerned with the psychology of the person loved, and with this person’s continuously changing mental life. And loving someone is a process, not a fixed state. Mutual love involves a shared history. This is why, if I have loved Mary Smith for many months or years, her place cannot be simply taken by her identical twin. Things are quite different with her Replica. If I have loved Mary Smith for months or years, her Replica will have full quasi-memories of our shared history. I have claimed that, if I do not love Mary Smith’s Replica, it is unlikely that the explanation is that I loved her particular body. It is doubtful that anyone has such love, or lust."

from Reasons and Persons by Derek Parfit