The reason this is rubbish has nothing to do with the capabilities of science. It's rather a question of what makes me me - what philosophers call the question of personal identity. The Raelians presume that an accurate copy of me, a copy that shares the same DNA, is the same thing as me. But it just isn't.

Part of the dodge that makes the cloning idea look half plausible is that we are led to imagine the clone taking on the parent's downloaded personality at the time of the parent's death. In this way the parent and the clone are not thought of together in the same space, but rather as one succeeding the other, the "I" being handed down the generations. But what if we imagine the parent's personality being downloaded into a clone, or even multiple clones, which then live alongside the parent. Can we really make sense of the idea that there are actually half a dozen people, all living at the same time, all of whom are really me? We don't treat identical twins as one person, so why should we treat my identical clone and me as one person?

One of the ways we can test whether it is nonsense to speak of clones with identical downloaded personalities as being the same person as the originating self is when we think about questions of responsibility.

Imagine, after I die, it is discovered that I had committed a terrible murder. Would my clone then be responsible? Would it be right to send him to prison? At the trial my clone would protest that it wasn't him but his parent who committed the crime. And he would be right - the conclusion of which is that whatever the physical and psychological similarities between clone and parent, the two can never be the same person. If this is the case then there can be no eternal life through cloning; simply, at best, a succession of different people who happen to look and behave the same way as me. What a nightmare.

But even if it were possible to live forever, would we really want to? And would human life really be as valuable if we were able to do away with the limits that define it? American philosopher Martha Nussbaum makes the point that it is the limitations of being human, in particular the limitation of our mortality, that gives meaning to what we are and much that we value. Indeed, a life that is without the possibility of death seems altogether more shallow in comparison.

In classical literature much is made of gods who fall in love with mortals. The beautiful goddess Calypso offers Odysseus a life on her island free from ageing and death. But rather than accept her offer of immortality Odysseus chooses to continue his dangerous journey, a journey fraught with risk and the possibility (and eventual certainty) of death, so that he might come again to his beloved Penelope.

In the world of the immortal ones there can be no such thing as risking one's life for the love of another. There can be no room for the heroism of sacrifice. No wonder the gods fall in love with mortals, for compared to the anaemic possibilities of immortal love the love of mortals is always going to be more passionate and intense. What, for example, becomes of the desire to protect and nurture another when welfare is guaranteed in advance? What sense can there be in the anxious and loving attention one pays to a fragile human baby if human life is invulnerable?

Advances in biotechnology and medicine are constantly pushing back the limits of our mortality, helping us live ever-longer lives. It's one thing to push against the limits, it's another thing entirely to imagine human life without any such limits. For it is our fragility that makes us what we are. And as Nietzsche argued against Christianity, the fantasy of never-ending life, of life without fragility, is not a celebration of the human but a disparagement of it.

But Nietzsche misunderstood Christianity on this point. For the aim of Christianity, as well as that of most of the world's mainstream religious traditions, isn't about living forever: it's rather about the transfer of interest from self to God. "We must divest ourselves of the idea that limitation implies something derogatory or even a kind of curse or affliction," argued Swiss theologian Karl Barth. Rather, Christianity speaks of dying-to-self. What this involves is wholly incompatible with the ego's obsessive desire to go on and on. For eternal life isn't living forever: it's a freedom that begins the other side of self-regard.

· The Rev Dr Giles Fraser is the vicar of Putney and lecturer in philosophy at Wadham college, Oxford.