For six weeks now we have been doing something difficult, and it is worth naming what it has cost, because tonight's question turns on it.
Every answer we have reached for has asked you to work it out by feeling. The victim: picture the person you are harming, and judge whether the wrong runs from you to them. The world: picture the worse one, and judge whether it is worse even with no one in it to be wronged. The claim: picture a future person with a rightful demand on us, and judge whether they have any standing to make it. Six episodes of holding your own moral sense up for examination and asking whether it can bear the weight. It is tiring. It is the kind of tiredness that makes a person want to hand the whole question to someone who knows what they are doing.
And tonight, someone offers to take it.
There are people who will tell you that this entire argument — victims and worlds and claims — was amateur work, informal reasoning of the kind you do at a kitchen table. They did it properly. Years ago, with models and data. They took the future — not a slogan about it, but the actual thing, every life that will be lived after us — and they put a price on it. They worked out, in plain figures, what a harm a hundred years from now is worth to us today. And that price is not confined to an academic journal. It governs policy. It sits inside the calculation behind every climate decision a government makes: how much to spend, how fast, what is worth protecting and what is cheaper to abandon. One number, doing the thing you have failed to do with your whole conscience for six weeks running: deciding what the future is worth.
And it is tempting. It tempts me too, after all this — a point at which the feeling stops and the arithmetic takes over. You would not have to judge by feeling whether a flood in 2200 is worth a tax today. Someone has measured it. You could simply defer to them.
This is Philosophy for Us — philosophy for everyone, no degree required.
This is the sixth of nine episodes. Last time we reached for something as substantial as a person's own claim — the standing of the future, a fair agreement struck across the generations, or a plain right to a decent life — and we watched both of those serious answers fail for the same reason: the person whose claim it is has not been born. Tonight we take up the offer to stop reaching altogether. We go into the economists' number, the one that promises to do our feeling for us, and we find out what is actually inside it.
I will tell you now what we find, because it is the whole hour and there is no reason to withhold it. That number is not pure arithmetic. Inside it is a moral judgment. The relief it holds out — here, the mathematics has taken the question off your hands — is a familiar manoeuvre in this argument. The ethical question did not disappear when the economists arrived. It was built into the number, where no one thinks to look for it.
Let me give the economists their due first, because the easiest way to defeat an argument is to attack a weak version of it, and their argument is not weak. It is the most serious attempt anyone has made to answer the question by calculation instead of by feeling. Before we find what is hidden inside it, you have to see why so many careful people adopted it and never turned back.
Here is the real problem they are solving. You want to do something about the future. But "do something" is not a policy. At some point an actual government has to decide whether to spend, say, a hundred billion now on a sea wall that will protect a city in the year 2150. To decide that, you have to compare two things that are hard to set side by side: money and effort spent now, against suffering avoided then — a century and a half later. How much present cost is a future rescue worth? You cannot act on the future until you can put the now and the then on a single scale and weigh them against each other. Every other answer we have tried this month gives you a feeling about the future. None of them gives you a scale. The economists built the scale. As far as I can tell, it is the only one anyone has ever built that turns a conviction into an actual decision.
And a scale needs a setting. That setting is what we are here to examine. Let me describe it with no numbers at all, because the idea underneath it is something you already accept.
A dollar in your hand today is worth more to you than the promise of a dollar next year. Not because you are impatient — there is a plain reason. Invest today's dollar and it grows; by next year it is a dollar and a little more. So a dollar promised for next year is worth slightly less than a dollar now — you would trade the promise for a little under a dollar today and consider it fair. That yearly gap is the whole idea. Carry it forward. A dollar promised in ten years is worth noticeably less today; in fifty years, less again; in two hundred years, a dollar of anything — a dollar of harm, a dollar of rescue — shrinks, in today's accounting, to almost nothing. A rounding error.
That shrinking is called discounting, and its speed is called the rate. That is all the rate is: how fast a thing loses worth, in today's accounting, for sitting further out in time. Fix the rate and you have fixed the exchange — you can finally say what a future flood is worth in today's money, and a government can finally act.
Now consider how much depends on that one setting. It is not a seminar opinion. It is the single choice the whole calculation turns on. Set it one way and a city in 2150 is worth defending now; set it the other way and the same city barely registers as worth defending. Which coastlines get a sea wall and which are quietly written off comes down to where that setting sits. The well-known disagreements — spend hard and fast, or steady and slow — almost all of them, examined closely, turn out to be a disagreement about this one number. Two honest people, with the same science, the same models, the same facts about a warming world, can reach opposite conclusions about what to do, and the only difference in their calculations is the rate. The science does not separate them. The number does.
So here, at last, is the relief I promised at the start. After six weeks of weighing victims and worlds and claims with nothing but your own conscience, here is a place where it all comes down to one clean figure that the specialists can argue out among themselves. You do not have to feel your way to it. You find the right rate, put it in, and follow the mathematics.
Which raises the only question that matters tonight. What kind of thing is that number? Because everything — every coastline, every hundred billion, every future life set against a present cost — depends on it. Before we trust it to do our feeling for us, we had better know what we are trusting. Is it a measurement — something you could in principle get right, the way you get the boiling point of water right? Or is it something else, presented as a measurement? That is what we turn to next.
So far the number sounds like a setting for how aggressive a policy should be: spend hard, or spend gently. That is how it is usually presented — a question of pace, of how much sacrifice a generation can bear. But keep looking, because underneath the question of pace is something larger.
Watch what the rate does when you apply it not to money but to people. Climate harm is not only dollars; it is floods, famines, lives cut short. So weigh a life. A death prevented next year, and a death prevented two hundred years from now. The same death — a person who does not drown, does not starve, keeps their whole life instead of losing it. Run the discounting on it exactly as you ran it on the dollar. The far-off death comes out worth a fraction of the near one. Not because the future person suffers less. They suffer exactly as much. The arithmetic simply marks their rescue down for being distant in time, the way it marked the dollar down — until, two centuries out, at an ordinary rate, a whole human life saved is worth, in today's accounting, about what you would pay to avoid a parking ticket.
I found this startling the first time I worked it through. The number that was going to do our feeling for us turns out to be doing something very particular. It is setting an exchange rate — how many lives later are worth one life now. And it sets that rate quietly, in a unit that looks like banking. No one at the table has to say the sentence aloud. No one has to stand up and say that a child in 2250 counts for a hundredth of a child today. They set the rate, and the rate makes the claim for them, in a form that sounds like mathematics.
Now, to be fair — and I want to be fair here in a way that matters for the rest of the hour — there may be a perfectly decent reason buried in this. Perhaps the future person really is, in some honest sense, easier to help, or already better off, so that counting their rescue for slightly less is not an insult to them at all. We will come to that. I am not going to let us condemn the economists as monstrous, because that is the easy move, and the easy move is wrong. Some of that mark-down will turn out to be legitimate.
But what we have found here governs the rest of the hour. A question you would have called as moral as they come — how much does a life later weigh against a life now — has been answered. Quietly. Definitively. It is answered every day, in the settings of the models that run the world's climate policy. And it is answered in a form no one experiences as a moral answer, because it appears as a decimal in a spreadsheet, in a column the ethicists are never invited to read.
So here is the question that divides the rest of tonight, and I want it as sharp as possible. That number — the one quietly setting the worth of a future life — where does it come from? There are only two honest possibilities. Either it is a measurement: a fact about the world that the modellers go and find, and could in principle get right or wrong, the way you measure the strength of steel before building a bridge. Or it is a choice: something a person decides, a judgment presented as technical, where "getting it right" is not even the kind of thing it could be, because there is no fact out in the world for it to match. One of those is specialists doing mathematics. The other is specialists making the exact judgment you have been struggling with for six weeks — and not telling you they have made it.
Let us find out which.
Here is the good news, and I mean it as good news, not as a setup. When you open the number up, a genuine part of it is exactly what it claims to be: honest measurement, with no ethics smuggled in. So let us take that part out and set it aside, in plain view, before going after the rest — because the people who declare that the discount rate is simply a moral fraud skip this step, and skipping it is how they go wrong.
The legitimate part is this. Suppose your descendants are richer than you — which, for most of human history, later generations have been. Now imagine passing a gift of, say, a thousand dollars across that gap. To you, a thousand dollars is real money; it might mean a month without lying awake over the bills. To a great-grandchild three times as wealthy, the same thousand is a pleasant dinner. The same banknotes, far less good done. So if you are counting up goods — wellbeing actually delivered — a dollar handed to a richer future person genuinely buys less of what matters than a dollar kept for a poorer person now. Marking the future dollar down for that reason is not an insult to anyone. It is simply true that money does more good where there is less of it. That part of the discount is legitimate. Keep it. The economists are right about it, and a serious person does not pretend otherwise.
Now watch what happens when we remove that legitimate part and look at what remains. Because there is a remainder, and the remainder is the whole matter.
Return to the life. Not the dollar — the life. A person two hundred years from now who drowns. Does the "they will be richer" reason explain marking that down? It does not. A drowning is a drowning. A lost life is everything that person had, whether they would have been rich or poor, and being three times wealthier does not make their death three times less of a loss to them. So the legitimate part — the richer-descendant part — does almost no work on a life. Remove it, and the mark-down on the future life barely shrinks. Something else is doing that work. Something the "they will be richer" story cannot account for.
That something has a name. Economists call it pure time preference. And once you set aside the technical language, it is the plainest and starkest thing in the episode. Pure time preference is the amount you mark a future person's wellbeing down for no reason other than that it comes later. Not because they are richer. Not because you are unsure they will exist. Simply — later. Their joy counts for slightly less because it is further off. Their suffering counts for slightly less because it has not happened yet. That, and nothing else, is what remains in the number once the honest part is gone.
And there it is. That is the moral judgment inside the number. Read the definition again and ask what kind of claim it is. "Wellbeing counts for less the further off it is" — is that a measurement? Is it a fact about the world you could go and check, the way you check the strength of steel? It is not. There is no instrument that reads it off. There is nothing in the world that makes it true or false. It is a judgment about how much a future person matters — a bare moral judgment — sitting in the one slot in the calculation that everyone treats as too technical to question. The specialists doing mathematics turn out to have made the very judgment you have been making with your conscience for six weeks. They just made it in a form that did not look like a judgment.
If you were with us earlier in the series for the episode on blame and free will, you will recognise this shape, and know not to trust it. There, the question that mattered — am I really to blame — kept looking like a fact for the physicists to settle, a question about whether the atoms could have gone differently. And it turned out that the thing you cared about was not settled at that level at all; it lived at a different level, one the physicists' instruments do not reach. The same thing is happening here. The discount-rate disagreement looks like a question for economics — a technical matter with a right answer the experts will find if they are careful enough. It is not. The part that matters is not something any amount of careful measuring will settle, because it was never a measurement. It is a verdict. And a verdict is the kind of thing someone has to own.
So notice exactly what we have done, because it reaches well beyond climate, and it is the part worth keeping. We took a number the whole world treats as purely technical — dull, settled, the experts' business — and we found a moral verdict inside it, in technical form. We did not need new data to find it. More data would never have found it — the look of a technical problem was itself what concealed it. We found it by asking one question the number's technical appearance is built to discourage: is this something you measure, or something you choose? Hold on to that question. It works on far more than discount rates, and before this series is out you will want it again.
But we are not finished, and here is where it would be easy to go wrong. Having found a choice sitting inside the number, the tempting thing now would be to make that choice for you — to say that if it is a choice and not a fact, then obviously the right choice is zero: count the future fully, mark no one down for being born late, done. That feels like where this is heading. It is not where I am going to take you.
So let me show you the actual disagreement, between two people who both saw exactly what we have just seen — that the number is a choice — and chose differently. Not a fool and a wise man. Two serious people, and the gap between them is a single decimal place.
On one side, Nicholas Stern, the economist who led the Stern Review, the report on the economics of climate change commissioned by the British government and published in 2006. Stern set the pure mark-down — the part that counts a future life for less simply because it is late — at almost nothing. A fraction above zero, and that fraction is not impatience; it is only the bare chance that there is no one there at all — that humanity does not continue, and the future life never arrives to be counted. Strip that out and his answer is zero: a life in the year 2300 weighs the same as a life now. Hear the case for it, because it is a strong one, and it is close to where your own sense already sits. You cannot look a person in the eye — even a person not yet born — and tell them their suffering matters less because of the date on it. Being born later is no more a reason to count for less than being born farther away. Stern's near-zero rate is simply the refusal to do that. It says: I will not mark you down for when you arrived.
On the other side, William Nordhaus, the Yale economist who spent much of his career on this and won the Nobel Prize in Economics in 2018 for his work on the economics of climate change, and who set that same number distinctly higher. Here you must not let me, or anyone, turn him into the greedy one, the man indifferent to drowned children — because that is a caricature, and the caricature is how the argument is lost. Nordhaus has reasons, and they are not "I care less about the future." There are two of them, and both have force.
The first is honesty about what we actually do. Look at how people really trade the present against the future — in every loan, every investment, every choice to spend now rather than save. Human behaviour runs at a positive rate; that is the rate the market pays, the rate revealed in what people actually choose, not what they profess in a seminar. Who are we, Nordhaus asks, to override the rate humanity lives by and impose one no one acts on? Set the rate from pure theory and you are not being moral; you are being presumptuous.
The second reason is harder, and it is the one that should give a supporter of Stern pause. Take that near-zero rate — count every future life fully, all the trillions of them stretching ahead — and run it honestly through the calculation. It does not tell you to be somewhat more generous. It tells you to sacrifice almost everything, now. If the future counts as much as the present, and there is an enormous amount of future, then nearly every dollar we spend on ourselves today — every comfort, every hospital that treats the living, every pleasure — should arguably be taken from us and invested for them instead. Almost no one believes we are required to live that way. You probably do not. So the near-zero setting, followed where it actually leads, arrives somewhere no one can live — the same shape we have met before in this series: a conclusion you cannot fault on its logic and cannot accept in your life. Nordhaus's positive rate is, in part, a retreat from that conclusion.
Now a third figure enters: the philosopher and economist John Broome, who has written at length on the ethics of climate discounting, and who says something that looks as though it settles the matter. Broome searched for any honest defence of the pure mark-down — any reason to count a future life for less merely because it is late that a clear-eyed person could actually hold. His verdict is blunt. Pure discounting, he writes, "is incredible unless it is combined with relativism, and relativism is itself most implausible." In plain terms: the only way to make "the future simply counts for less" hold together is to rest it on a theory of value that almost no one can accept once they see it clearly. The bare preference for now over later, offered as a reason, does not survive examination.
You can feel where that seems to lead. If the pure mark-down cannot be defended, then Stern wins: set it to zero, count the future fully, and we are done. And this is exactly the point where I stop, because that would be slipping in the conclusion I promised not to slip in. Look again at what Broome actually refuted. He refuted the justification — the idea that lateness, on its own, is a good reason to count someone for less. He did not refute Nordhaus's two arguments, because neither of them is that. "Use the rate people actually live by" is not a claim that the future is worth less; it is a claim about who has the standing to prescribe rates. "A rate of zero would demand a level of sacrifice no one will accept" is not a claim that the future is worth less; it is a claim about what we can really be required to do. Broome removes the least defensible argument for a positive rate. He leaves the other two standing. A serious person, after Broome, can still hold the rate above zero — just not by saying the future matters less.
So here is where we actually land, and it is not on a number. There is no setting of this rate that is simply correct. Stern's near-zero asks you to accept a level of present sacrifice almost no one can live with. Nordhaus's positive rate asks you to use a number that, followed far enough, quietly counts a far-off life for less. Each one accepts a cost the other cannot. Choose either, and you have not solved a measurement; you have made a moral choice and written it in the language of finance. And now the whole thing is visible. They call it valuing the future — and that word is doing two jobs at once. There is value in the accountant's sense: a price put on a thing. And there is value in the moral sense: what something is genuinely worth, who genuinely counts. The discount-rate disagreement looks like the first kind. It is, all the way down, the second. The accountant's word had carried the moral question inside it the entire time.
Let me set down what tonight has actually given you, because it is a real thing and you can carry it with you — and then I have to tell you the difficulty with it.
The good thing first. You came in tonight believing — or half-believing, hoping — that somewhere among the answers we have tried there was a place where the feeling stopped — where the specialists had taken the unbearable question off your hands and turned it into arithmetic. And now you know how to examine that work and find what they actually left there. You take a number the whole world treats as purely technical — dull, the experts' business, nothing to see — and you ask it one question its technical appearance is built to deflect: are you something we measured, or something we chose? And when the answer is "chosen," you have found a moral verdict that someone placed in a slot no one thinks to examine. You can do that now, and you cannot un-learn it. It works on far more than this one number — and that turns out to be less of a relief than it sounds.
Now the difficulty, and it is the shape of the whole series arriving at once. That was your last way out. Every week you have looked for something solid to rest the obligation on, and every week it has failed: the victim who turned out not to be there, the world too cold to live in, the claim with no one to hold it. And the number — the experts' number — was the final exit, the one that promised you would never have to feel your way to the answer, because someone already had. We went in, and we found your own conscience there, in the form of a decimal. The exit does not lift the weight off you. It hands it straight back. There is no one to pass this to. The mathematics you hoped would decide it is the same choice you have been refusing to make, written in a form that let everyone pretend a machine had made it.
And notice — I will say it once and lightly, because you already know it — the feeling has not moved tonight either. We did not even put a new ground under it this week; we removed your last hiding place. It stays exactly where it has been since the first night, exactly as heavy, still refusing to let you dismiss the future. Only now there is nowhere left to send it. I am not handing you a rate, by the way — not Stern's, not Nordhaus's, not one of my own. The honest part of the discount still stands; a dollar really does do less good for a richer future person, and I am not taking that back. What I will not do is hand you the other part — the worth of a life later against a life now — with a number on it. That part is a choice. It was always going to be yours.
We are six episodes in, and there is still no settled answer. Every week so far has had the same shape: what do we owe, and to whom. Next week the question changes. Not what we owe the future, but how you are supposed to act toward it at all — which turns out to be a problem with its own difficulties, and sharper ones than you would expect. That is where we go next.
For now, take one thing out the door with you — and do not trust it because I said it; trust it because you watched it happen in a case where nothing is at stake but the seeing. Choose something with nothing to do with the climate. Picture a town with one ambulance. One. Someone has to decide where it waits when it is not on a call — because where it waits decides who it can reach in time. So a planner writes a formula. Sensible, technical, the kind of thing you would never think to question. It weighs how fast the ambulance can reach each part of town. And inside that formula is a single number — a number that says how much a minute saved for the outlying farms counts against a minute saved for the crowded town centre. Find that number. Because the moment you change it, the ambulance moves — and with it, quietly, who tends to be reached in time and who does not. Who lives a little more; who dies a little more. It looks like logistics. It looks like a matter for the traffic engineers. Ask it our question: is this measured, or chosen? No better map of the roads will ever answer it, because it was never about the roads. It is a judgment about whose minutes weigh more — sitting in a slot marked technical, where no one thought to look. Run that, at your own kitchen table, on your own town. And when you have found the moral judgment inside the logistics, you will know what the economists did with the future, and why no number was ever going to do your feeling for you.
Thanks for listening. I'll see you next time.