I wrote about this for Zvi last month.

However, I must express my perennial doubt that cryonics actually permits personal survival. I lean towards the view that at best it permits the creation of a copy of the dead person.

Consider Ray Kurzweil's desire to resurrect his father, using only scraps of information such as DNA, photographs, and the memories that living people still have about his father. Logistically, this is not very different to "resurrecting" Sherlock Holmes, or any other fictional character. What is being contemplated, in either case, is the de novo creation of a conscious being, with an identity based on memories of a life that it never lived.

As we do not know the true nature of memory, and we especially do not know what is involved in the technological fabrication of a lifetime of memories, it may be that there is no epistemic difference, no subjectively discernible difference, between true memories and implanted memories. In pondering the idea that the conscious self may indeed be a holistic "quantum" subsystem of the brain, with sharply defined boundaries in space and time, I have wondered whether there may be a true form of memory, which can only be laid down by living through the experience, because the memory information has to get dynamically enfolded into the state of this subsystem, and the only way for that to happen might be for that subsystem to endure through time the boundary conditions which correspond to actually having the experience.

However, let us assume the contrary - that there is no such thing as long-term memory that is guaranteed to be veridical, that the memories of an individual mind have the same modularity and replaceability that the "memory" in a computer possesses. Nonetheless, there is an obvious sense in which a newly minted conscious being, that somehow gets supplied with a readymade lifetime of experience at its inception, did not actually live that life. Even people who insist that a copy, restored from a backup of someone's mind, is that person, will be able to distinguish conceptually between one physical instance and another physical instance of that person, and should readily admit that the second instance did not physically live through the experiences recorded in the memories which it inherited from the first instance.

The relevance of this for cryonics is that I suspect that the resurrected person stands in precisely the same relationship to the one who died, as a copy does with respect to its original, in a scenario of mind uploading. This is most obviously the case if one imagines people in cryonic suspension being brought back via simulation of their brains in some nonbiological way, but it may even hold for people whose frozen brains are nanotechnologically restored to functionality without disassembly or nonbiological substitution of parts, brains which are simply set in motion again through a highly coordinated process of local thawing and local repair.

This is the case in which insisting that the continuity of personal identity has been interrupted is most counterintuitive, because in the whole process from life, through death and cryonic suspension, to restoration, the same brain sits in the same skull. I would compare it to a situation in which a mirror is shattered but all the pieces remain in the frame and next to each other. You might be able to reattach them to each other, and get a whole mirror again, but in the interim, the mirror was shattered. I am very skeptical that true identity survives a process as microphysically disruptive as cryonics. I'm not 100% convinced that it can't, but I lean towards the view that cryonics is simply preserving fantastically detailed records about people who are actually dead, and therefore at best providing raw materials for the creation of new people who might think, at first, that they are the person found in their memories, but who did not actually live that life, just as the freshly minted person who has been supplied with the memories of Sherlock Holmes never actually lived at 222-B Baker Street or engaged in a death struggle with Professor Moriarty.

Those are my views on cryonics. Life extension and rejuvenation are a far more straightforward issue: they are to be desired almost unconditionally, and resistance to the very idea is mostly due to weariness with life; see the link at the start of this comment. For someone who has lost their physical youth, choosing to regain it should be an immediate "yes". However, in the real world of the present, there is no such option, so reaching for rejuvenation turns out to involve staying alive in a nonrejuvenated form for an unknown period of time, and perhaps taking risks with half-baked and untested biotechnology when rejuvenation does begin to be an option. Apart from the cryonics option, I think the most you can do is encourage them to be healthy and live healthily, something which most people want to do anyway, and encourage them to think about the possibility that they will live to see a second youth become possible. You can't promise them anything more than that.