The “repugnant conclusion” that an Oxford philosopher couldn’t escape

Thanos did not study under Derek Parfit. In part, this is because Thanos was a Titanian Eternal warlord, an alien born on a moon of Saturn. Parfit, however, was a British moral philosopher who taught at the University of Oxford starting in the 1970s. But I also know this because Thanos and Parfit came to very different philosophical conclusions.

Thanos’ logic in the Marvel movies — spoiler alert for Marvel movies before 2020 — was that, “If life is left unchecked, life will cease to exist…It needs correction.” Given that the Universe’s resources are finite and life seems to show no signs of limiting itself, he thought we needed to step in and increase the quality of life. Thanos argued that it would be better to have fewer people enjoying more, rather than lots of people “enjoying” barely anything.

In this week’s Mini Philosophy interview, I spoke with David Edmonds. Edmonds co-created one of the world’s most popular philosophy podcasts, Philosophy Bites, and he has also recently written a book on Parfit, who came to the opposite conclusion from Thanos. But he wasn’t happy about it — his “repugnant conclusion” bothered him right until the end of his career.

So, what is the repugnant conclusion, and why was it such an intractable thorn in the side of one of the greatest modern philosophers?

The repugnant conclusion

The repugnant conclusion is a kind of self-induced reductio ad absurdum where Parfit was forced to accept its arguments on the basis of certain first principles. His first principle in this case was the belief that a good action is one that creates the most happiness. A good world is one with the most overall happiness. This is a basic utilitarian principle, and while it somewhat avoids the issue of “What does happiness mean?” I suspect most people operate a version of this principle in their everyday dealings with other people.

The problem, though, is a mathematical one. This is how Edmonds describes it:

“Imagine a world a bit like our world with, let’s say, eight billion people in it. And let’s say everybody has a good life. There’s nobody in severe destitution. We all have nice lives. We spend our time reading philosophy books and playing golf. Whatever. And we’re all very happy.”

“Now, imagine a world billions and billions of times bigger than that, where there are trillions of people whose lives are only just better than nothing. Which is better: the former world where we’re all playing golf and enjoying ourselves, or the latter world where there are trillions of people whose happiness levels are kind of only just above zero on the ledger?”

Parfit argues that we must conclude the latter world is “better” because it has more overall happiness for more people.

An unavoidable endpoint

Parfit never came to peace with the repugnant conclusion, but he couldn’t find a way to avoid it. What were the options? We could deny that more people make the world better — but that leads to the counterintuitive view that creating happy lives has no value at all. A (just about) happy person is a good thing, so why not a trillion (just about) happy people? Another option is to argue for an upper limit to population value — a kind of diminishing return on human happiness. But that quickly runs into arbitrariness: Why would adding a happy person be good up to one number, and then suddenly not?

Edmonds’ answer is to examine the happiness calculus more closely. Under traditional utilitarianism, “happiness” is defined as having more pleasure than suffering. Edmonds thinks there are two problems with this.

First, Edmonds argues that “there is an asymmetry between happiness and suffering. You can’t put pain on one side and happiness on the other. Philosophers make a mistake in thinking that there’s suffering over there and happiness on the other side, and I don’t think that’s the way we should think about it.”

Second is that unavoidable issue: Who defines happiness, and by what metric? “It’s interesting that when we’re happy, we’re not aware that we’re happy. It’s when we’re doing something else, and it’s only in retrospect that you look back and say, ‘Yeah, I was in the flow.’ Happiness is a kind of flow.”

If it’s often difficult for us to measure our own happiness in a timely and meaningful way, good luck trying to measure happiness on a universal scale.

The very real future

As with all good philosophical thought experiments, the repugnant conclusion has real-life implications. It’s something we grapple with as a society, but also in policy and the courts.

For example, when it comes to climate change, the question becomes whether we should prioritize policies that benefit future generations, even if those lives are marginal or precarious. Within the idea of “AI long-termism,” groups like OpenAI and the effective altruists argue that we must preserve civilization itself — not for us, but for the vast numbers of people who might one day exist. And when we consider global development policy, should we direct resources toward lifting current lives out of poverty, or toward ensuring a larger, stable population can exist in the long run?

Parfit’s thought experiment lingers behind these questions. It’s about the quality of a life set against quantity. It’s about the value of those alive today against those who might be alive in the future. As with all ethical questions, there are no easy answers, but we might be forced to find and live by an answer, nonetheless.