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By Ben Davies

Nicholas Agar has come up with an interesting objection to the idea of radical life extension (RLE), a prospective set of technologies that aim to significantly extend our lifespans by preventing us from ageing, thereby making us “negligibly senescent” (NS). Agar recommends a ‘species-relativism’ about value that he thinks speaks against RLE. Essentially, things that have value for one species may lack value for another species. I can acknowledge that a dog is very happy, perhaps happier than I am, but still be glad that I am not a dog because certain things that I currently value are only valuable to humans. Crucially, this argument is supposed to apply if we improve rather diminish our capacities. A hyper-intelligent alien might find the great works of human literature to be trite and unimaginative, but it would be a genuine loss for me to come to such a conclusion about something that I currently value by enhancing my own intellect, even if I would have objectively more valuable experiences in such a state. Agar suggests that RLE will make us ‘posthuman’, and that species-relativism shows that this is a bad thing.

Clearly, the argument only works if Agar can demonstrate that RLE makes us posthuman, and so if we adhere to Agar’s definition of what it is to be in, and thus to leave, the human species. He starts by defining species as a “group of populations whose members are capable of interbreeding successfully and are reproductively isolated from other groups”. To leave a species, then, one must face “reproductive barriers” between oneself and members of the species. Although we are very similar to gorillas, we cannot reproduce with them, and so we are separate species.

The effects of RLE that constitute reproductive isolation are fourfold. First, Agar believes that long lives will make us extremely risk averse. We will have much more to lose through death (since we would have much more life ahead of us). So we will eschew many activities that humans enjoy, namely activities that would lead to sudden death (e.g. a car crash; drowning; being struck by lightning) rather than gradual death (accumulated damage that will be reparable by RLE techniques e.g. smoking; eating fatty food). Our interests will thus realign towards gradually damaging activities, and away from potential sudden damage. We should note that the activities that will be too risky for NS persons are things that we do without a thought today, such as going for a drive or swimming in the sea; NS persons will thus retreat from the world.

Second, Agar thinks that NS persons will be much more prone to illness; their unchanging genetic makeup will mean that their immune systems cannot adapt to viral, bacterial and fungal evolution, as accompanied by genetic reconfiguration in sexual reproduction. As such, NS persons will avoid contact with others, for fear that they will be exposed to an illness that their bodies cannot fight off. Third, people with indefinite life spans are less likely to want to engage romantically with people who will grow old and die. Finally, for various motivational and biological reasons, Agar suspects that NS persons will be unwilling, and possibly incapable, of having children. These aren’t the only costs of RLE, but they are the four that seem to form Agar’s argument for NS persons being posthuman.

To see why Agar thinks these problems divorce NS persons from humanity, consider a further aspect of his species-theory, that “reproductive isolation can be a partly psychological matter”. He notes that while humans and Neanderthals might have had significant chromosomal differences that made them unable to reproduce, there were probably also substantial psychological barriers that made them mutually unattractive. Agar suggests that we can distinguish between psychological aversions that are cultural, such as racism, and a “hardwired” aversion to “things that appear human but aren’t really”, an indicator of a suboptimal or even useless reproductive partner.

Agar defines reproductive compatibility in slightly broader than biological terms, as being partly “about having offspring that can be acknowledged as children and successfully raised to adulthood”. If the four effects he describes are accurate, it does seem unlikely that a NS person would reproduce with someone who hadn’t undergone RLE. Agar seems to think that this makes NS persons part of a different species.

There are problems with this claim. Psychological barriers can contribute to biological species-difference, by creating divides that lead to reproductive bifurcation of a species, or can simply warn us against attempting to reproduce with sub-par partners, as in the human-Neanderthal divide. Yet it seems implausible that we could separate groups into distinct species based solely on psychological barriers, so long as there remains a capacity to reproduce. As Agar acknowledges, various cultural barriers to reproduction – such as racism – do not lead us to identify multiple species. NS persons’ hermetic tendencies, and their preference for people of their ‘own kind’, do not seem sufficient to establish a species distinction, even if that means there are two groups who do not reproductively interact with one another.

This issue becomes clearer if we consider Agar’s explanation of those who are currently reproductively isolated. One example he offers is those who exist in self-imposed isolation, such as members of monasteries. These individuals qualify as human because of “potential or counterfactual reproductive connections with other humans”. When explaining this monastery example, Agar focuses on what individuals could do. However, when looking at the NS, he focuses on what will happen. Making the account consistent, in either direction, undermines his argument that RLE will lead to posthumanity.
Consider first what Agar would have to say if he considered actual behaviour in both instances. Assume he is right that NS persons would have psychological reasons to avoid reproduction, and even contact, with those of a normal lifespan. It is equally true that a committed monk, or lifelong prisoner, will not have any reproductive activity with the majority of humans.

The more plausible option, which Agar takes, is to acknowledge that the monk is still capable of reproduction, were his circumstances to change. The monk’s circumstances may include his desires and preferences – he may consider it morally wrong for him to reproduce, and this would need to change in order to end his reproductive isolation. Yet the very same consideration applies to the NS person. As Agar defines her, she is very risk-averse, has no interest in reproducing at all, let alone with someone whom she will outlive by centuries. But she still seems capable of reproduction, in the broader social sense that Agar uses. She could decide that the emotional and mortal risks are worth it. Even if we accept Agar’s assertion that she almost certainly won’t, we need some reason to focus in this case on what the NS person will do, when we see that the most reasonable option in the monk case is to look at counterfactuals of what he could do.

Agar has a final argument up his sleeve. He claims that RLE “is more likely…to result in beings who represent some kind of new beginning” because NS persons’ “evolutionary futures are less likely to depend on any contributions that they might make or might have already made to the human species”. Perhaps Agar would note that the monk will die without passing on any genetic material, and so no ongoing group barrier is established. Conversely, NS persons continue over time; if their preferences remain stable, psychological reproductive barriers will divide humanity into two well-defined groups. At this point, Agar might suggest, we have separate species.

We can imagine reproductively isolated groups that are importantly different from the ones Agar considers. Consider a religious order much like Agar’s monastery, aside from a couple of details. The order is not gender-exclusive, and members are not celibate, but may only reproduce with one another. We thus have two groups: the order and the rest of humanity. The evolutionary prospects of members of the order do not depend at all on sexual reproduction with outsiders, and vice versa.

I think it is clear that members of the order are still humans. Yet their reproductive isolation differs in no significant way from that of NS persons. It is self-imposed, based on certain uncommon preferences, involves social isolation, and is potentially reversible. Absent a stronger argument to the contrary, I conclude that a negligibly senescent person, even if they were isolated in all the ways Agar predicts, would remain human. This conclusion does not deny the serious costs of Agar’s suggestions, but it does mean that even if NS persons enjoy very different pleasures from those we enjoy today, those would still be human pleasures. So Agar cannot apply his species-relativist argument to this form of enhancement.


N. Agar (2010) Humanity’s End. MIT.

N. Agar (2010) ‘Thoughts about our species’ future’ in Journal of Evolution and Technology 21(2).