Taking a stand for life, revisited
If I chose to be flippant, I might argue that the number of people who say they would not take advantage of life-extension technology if it were available refutes the proposition that life extension would reduce the death rate to near zero and thus require us to forbid new births so as to avoid a population explosion. However, I really doubt that this "die-and-let-live" philosophy (as I call it) accurately reflects what will happen.
Observed behavior is a much better predictor of future behavior than verbal statements are. Many of the dramatic medical innovations of the twentieth century were at first viewed with suspicion as hubristic meddling with nature, but later widely accepted once people got used to them. (In vitro fertilization, stem-cell research, and even contraception are still at various stages of this acceptance process.) There are, of course, still people who reject arbitrarily-defined categories of "too-artificial" medical technology even when necessary to save their lives, but these people are few in number and mostly members of fringe religious groups. As anti-aging technologies become available, I believe the same process of general acceptance will happen -- probably quite quickly.
There is a vast difference between contemplating aging and death in the abstract, and confronting them as immediate realities. In fact, almost all humans in situations where they are faced with the possibility of imminent death react by taking any necessary measures to survive. I can't prove it, but I think that almost any person on his deathbed, if offered access to treatment that would restore his health and allow him to return to a normal life, would seize the opportunity, not wave it away.
At 46, I am aware of a slight decline in stamina and energy compared with how I was a quarter-century ago. The loss in vitality is small, and the huge increase in mental acuity due to a quarter-century of added life experience far more than makes up for it. But who can argue that it wouldn't be better to have the best of both worlds? And how much more true this becomes as the decline accelerates over time! I have seen very elderly people who have become enfeebled, slow, and vulnerable, their eyesight and hearing weakening, even their minds starting to slip despite the advantage of almost a century of experience. Who would truly choose to be in that condition if it were avoidable? If identical afflictions were to descend suddenly on a young person, it would be considered a terrible tragedy. Why should others have to endure them merely because they happen to have been alive a few decades longer?
It is sometimes argued that aging and death are part of nature and are hard-wired into our species, as if this meant that it would somehow be illegitimate for for us to use technology to resist them. But malaria, polio, syphilis, and so forth are equally part of nature, and many degenerative and hereditary diseases are hard-wired into human biology (or at least into the biology of some individuals), and no one argues that we should refrain from using technology to free ourselves from those scourges.
What matters is not whether a phenomenon is natural or not, but whether it is harmful or beneficial to us. That death is harmful to an organism is so obvious as to be practically tautological, and does not even need to be argued in most contexts.
I enjoy the experiences life has to offer. My death would mean that I would no longer be able to have those experiences. It would also rob me of the ability to observe the future development of humanity, about which I am intensely curious. Therefore death is objectively bad. Some may differ, but I suspect most people's view of the matter would be closer to mine.
The folkloric heritage of our culture always treats efforts to defeat death and live forever as foolish hubris inevitably leading to a bad end. I think this heritage colors much of the reflexive negative reaction to the idea.
My interlocutor in the Fight Aging discussion argued against anti-aging research on the grounds that other medical issues should have a higher priority. I think I succeeded in refuting this view, but even if it were true, so what? The argument that no work should be done on a given problem because some other problem is more important leads inexorably to the position that no work at all should be done on anything other than whatever we decide is the one single most important problem facing us. Science has never worked that way, and we would be far worse off today if it had.
There is, of course, the issue of belief in an afterlife, which often comes up in discussions of this subject. To renounce the chance to extend one's lifespan in the hopes of experiencing some sort of afterlife is, to put it charitably, a gamble on extremely dubious odds. It is impossible to prove any typical religious claim of an afterlife (Heaven, reincarnation, etc.) to be false -- like most religious propositions, these claims are designed to be impossible to prove true or false. However, there is no evidence whatsoever that an afterlife exists, and plenty of evidence that a reasonable person would take as arguing strongly against the idea. If the part of the brain that performs a particular function is seriously damaged, that function is lost or compromised. It doesn't "go" anywhere; the brain simply loses the ability to do it, though it may sometimes manage to reconfigure itself to restore some of the lost capacity. (As Sam Harris points out here, believers in Heaven who lose a particular mental function presumably do not believe that that function has gone to Heaven ahead of the rest of their minds. I don't think reincarnationists believe that a lost function gets reincarnated all by itself, either.) People with severe damage to the parts of the brain that sustain consciousness lose the ability to be conscious, though if the cerebellum is undamaged their heartbeat, breathing, digestion, and other signs of organic life may continue indefinitely. The obvious conclusion is that when the entire brain ceases to function at death, all the activities that were going on in that brain simply come to a stop. There is no evidence for any mechanism whereby they could somehow continue without the brain, any more than programs running on a computer can continue to run in some disembodied fashion if the computer is destroyed.
It seems obvious to me that the belief in an afterlife developed as a form of comforting self-delusion in the face of the inevitability of death, and that in most people this belief is far shakier than their verbal claims about it would suggest. Most people whose survival is threatened fight hard to stay alive; most people mourn when someone they love dies. These are not the responses of people who truly believe that death is merely a gateway to another, more pleasant existence.
Finally, I have to say that there is something I find repulsive in the attitude of abject submission to the blind dictates of nature -- the feeling that, because biology mandates that we should age and die, it is somehow noble or our duty to accept it. No one would admire, say, subjects of a totalitarian state who adopted a similar craven attitude toward their rulers, making a virtue of submission and obedience to the arrogance and cruelty of another. All the great advances in human freedom have depended on people who rejected this mentality. Apparently some of our technological advances will depend upon them too.