Imagine for a second that the entire human race has gone extinct, with the exception of one man.

There is no hope for humankind to continue. We know, as a matter of certainty, that when this person dies, so too does the human race.

Got it? Good. Now, imagine that this last person spends their remaining time on Earth as an arbiter of extinction. Being themselves functionally extinct, they make it their purpose to eliminate, painlessly and efficiently, as much life on Earth as possible. Every living thing: animal, plant, microbe is meticulously and painlessly put down when this person finds it.

Intuitively, it seems like this man is doing something wrong. But according to New Zealand philosopher Richard Sylvan (though his argument was published under the name Richard Routley before he took his wife’s name when he married in 1983), traditional ethical theories struggle to articulate exactly why what they’re doing is wrong.

Sylvan, developing this argument in the 1960’s, argues that traditional Western ethics – which at the time consisted largely of variations of utilitarianism and deontology – rested on a single “super-ethic”, which states that people should be able to do what they wish, so long as they don’t harm anyone – or harm themselves irreparably.

A result of this super-ethic is that the dominant Western ethical traditions are “simply inconsistent with an environmental ethic; for according to it nature is the dominion of man and he is free to deal with it as he pleases,” according to Sylvan. And he has a point: traditional formulations of Western ethics have tended to exclude non-human animals (and even some humans) from the sphere of ethical concern.

Traditional formulations of Western ethics have tended to exclude non-human animals (and even some humans) from the sphere of ethical concern.

In fairness, utilitarianism has a better history with considering non-human animals. The founder of the theory, Jeremy Bentham, insisted that since animals can suffer, they deserve moral concern. But even that can’t criticise the actions of our last person, who delivers painless death, free of suffering.

Plus, most versions of utilitarianism focus on the instrumental value of things (basically, their usefulness). Rarely do we consider the fact that when we ask “is it useful?” we’re making an assumption about the user – that they’re human.

Immanuel Kant’s deontology begins with the belief that it is human reason that gives rise to our dignity and autonomy. This means any ethical responsibilities and claims only exist for those who have the right kind of ability: to reason.

Now, some Kantian scholars will argue that we still shouldn’t treat animals or the environment badly because it would make us worse people, ethically speaking. But that’s different to saying that the environment deserves our ethical consideration in its own right. It’s like saying bullying is wrong because it makes you a bad person, instead of saying bullying is wrong because it causes another person to suffer. It’s not all about you!

Sylvan describes this view as “human chauvinism”. Today, it’s usually called “anthropocentrism”, and it’s at the heart of Sylvan’s critique. What kind of a theory can condone the kind of pointless destruction that the Last Man thought experiment describes?

Since Sylvan published, a lot has changed – especially with regard to the animal rights movement. Indeed, Australian philosopher Peter Singer developed his own version of consequentialism precisely so he could address some of the problems the theory had in explaining the moral value of animals. And we can now pretty easily say that modern ethical theories would condemn the wholesale extinction of animal life from the planet, just because humans were gone.

But the questions go deeper than this. American philosopher Mary Anne Warren creates a similar thought experiment. Imagine a lab-grown virus gone wrong, that wipes out all human and animal life. That would be bad. Now, imagine the same virus, but one that wiped out all human, animal and plant life. That would, she thinks, be worse. But why?

What is it that gives plants their ethical status? Do they have intrinsic value – a value in and of themselves, or is their value instrumental – meaning they’re good because they help other things that really matter?

One way to think about this is to imagine a garden on a planet with no sentient life. Is it better, all things considered, for this garden to exist than not? Does it matter if this garden withers and dies?

Or to use a real-life example: who suffered as a result of the destruction of the Jukaan Gorge – a sacred site to the Puutu Kunti Kurrama and Pinikura people? Western thought conceptualises this as wrong to destroy this site because it was sacred to people. But for Indigenous philosophical traditions, the destruction was a harm done to the land itself. The land was murdered. The suffering of people is secondary.

Sylvan and others who call for an ecological ethic, believe the failure for Western ethical thought to conceptualise of murdered land or what is good for plants is an obvious shortcoming.

This is revealed by our intuition that the careless destruction of the Last Man on Earth is wrong, even if we can’t quite say why.