My obligation not to harm existing people seems urgent and non-negotiable, but the harm I might be inflicting on future people is easy to ignore. This is not necessarily evidence of moral failing. There are good reasons why we discount future people. But are they good enough?
One reason we discount future people is optimism. We assume future people will be better-off, and their interests will align with ours. If we create a stable liberal democratic society for ourselves, then our descendants will be healthier, wealthier, longer-lived and enjoy richer lives.
Like the great American 20th century political philosopher John Rawls, our only ethical question is then: How much better-off should we leave our descendants?
Recent developments such as climate change and financial collapse challenge this cosy optimism. We no longer assume we will leave future people better-off, or even that we can.
We realise they might be worse-off because we have looked after ourselves.
We also ignore the future because it lacks the second-personal dimension that is central to our moral lives. If I ran over your foot, I would need to justify myself to you. But actual future people don't travel back in time to demand satisfaction.
Finally, we ignore the future because there is no actual future, only innumerable possibilities. Any specific scenario might never happen. Even claims about probabilities are easily disputed (consider the controversy surrounding the reports of the Intergovernmental Panel on Climate Change).
But uncertainty shouldn't give us license to ignore future people altogether. We don't know we are harming them. But we cannot be confident that we are not.
One solution is to imagine credible futures - things that might well happen - and ask how we could justify ourselves to their inhabitants. This is not a perfect method but it is better than nothing.
In my book Ethics for a Broken World, I imagine a future broken world where resources are insufficient to meet everyone's basic needs, where a chaotic climate makes life precarious, and where our affluent way of life is no longer an option. Philosophers in that broken future look back in disbelief at a lost age of affluence, and try to understand the ethical thinking of affluent citizens such as ourselves.
This future is credible. No one can reasonably be confident it won't happen. It involves no scientific impossibilities or implausible expectations about human behaviour.
Why imagine possible futures? One role is motivational. Specific futures introduce second-personal urgency. Once we ask how actual people would think about us, we face the challenge to justify ourselves to them.
Credible futures also expose the arbitrary contingency of our own ethical thinking. To take one example, consider the importance we attach to the notion of rights. For us, a right is something that must be guaranteed to everyone. But if future people cannot even keep everyone alive, how will they think of rights?
In my book, I suggest that, in a broken world, even the most democratic societies will need to decide who lives and who dies. They will need to implement survival lotteries. To implement such a lottery now would be a monstrous violation of rights. But, in a chaotic climate, lotteries might be necessary to respect rights. How else can we fairly decide, say, how to balance freedom against survival? Perhaps, for future people, a right will be not a guarantee of survival but an equal input to collective deliberation, and then an equal chance to live or die.
Hypothetical survival lotteries are morally unsettling. But the prospect of an actual future lottery raises a more urgent practical question. If our descendants may need to institute a survival lottery, can we still insist on any guarantees for ourselves?
Unless we are prepared to insist that we are more valuable than future people - that we somehow deserve the winning tickets in a lottery across the generations - we cannot separate abstract discussion of life in a credible broken future from urgent decisions about how we should live now.
Tim Mulgan is a Professor of Philosophy at the University of Auckland. He is the author of four books, including Ethics for a Broken World. His inaugural lecture, Ethics for Possible Futures, will be held at the university on Wednesday, September 18.