Incompatibility of moral realism and time discounting

Epistemic status: Something I thought about a lot, but have not talked to many people about. I would not be very surprised if I will be convinced that this argument is flawed.

In this post, I am going to make the argument that moral realism is not compatible with time discounting. I am going to use special relativity in the process, but you don’t need to be familiar with physics to follow along. This post is based on an essay I wrote for university, and I will use it for my master’s application. So if any of you (especially with an understanding of metaethics) have any critique or feedback, I would be happy to receive it.

Relevance for EAs

Whether you use time discounting in your decision-making or not has huge practical implications. If you use time discounting, Longtermism is hard to defend. If you don’t use time discounting, Deontic strong Longtermism seems plausible.

As this is an argument against the compatibility of moral realism and time discounting, it is most useful for EAs, who have some confidence in both. If you don’t fall into that category, I hope you find it interesting too, but I don’t expect you will update your moral views by a lot.


I am first going to explain key features of time discounting, moral realism, and special relativity. Feel free the skip any of those explanations, if you have a solid grasp of these topics. If you want to get straight to the argument, just read from “Thought experiment” onward.

Time discounting

In the context of economics, time discounting describes the relative valuation of receiving a good or service on an earlier date, compared to a later date. This means that according to models that include time discounting, people would prefer, all else equal, to receive a pleasurable good or service rather today than tomorrow. This must not be confused with inflation, which leads to goods and services being higher priced at a later time, or the opportunity cost of having, let’s say, a factory working one day earlier and making a profit for one more day. Time discounting means that given the choice to receive the same benefits for the same price with the same consequences today or tomorrow, people prefer having them today, over tomorrow.

Some ethical theories also have time discounting. Jeremy Bantham names “propinquity or remoteness” as one of the categories, whereby “pleasure and pain [is to be] considered [...] more or less”(Bantham,1780,30).

This must not be confused with caring less about the future because one can have more influence or more certainty about events that occur sooner rather than later. According to moral theories with time discounting, just because an event is further in the future, it is less important, even if one could influence it just as easily or just as confidently.

Moral realism

Moral realism is the position that moral claims (for example “Anna should not sell Christoph`s car”) can be true or false, like regular claims about the world (like “Elsa is taller than Anna”).

One implication of moral realism is that there is some correct moral theory. With “correct moral theory”, I mean some method by which the truth value of any moral statement can be derived.

The most cumbersome formulation of the correct moral theory would just be a list of all possible moral statements, labeled true and false. Most moral philosophers think that there is a more parsimonious formulation of the correct moral theory, in the form of some rule or set of rules, from which the truth value of every moral statement can be derived.

Another implication of moral realism is that moral claims are true or false for everyone. This does not mean that “Anna should not sell Christoph`s car” automatically implies “Christoph should not sell Christoph`s car”. But if “Anna should not sell Christoph`s car” is true for Anna that also means that “Anna should not sell Christoph`s car” is also true for Christoph and everyone else. Just like “Elsa is taller than Anna” does not imply “Elsa is taller than Christoph”. But if “Elsa is taller than Anna” is true for Anna that implies that “Elsa is taller than Anna” is also true for Christoph and everyone else.

This might seem trivial, but it actually puts some constraints on what the correct moral theory might be. For example, the correct moral theory cannot be “Everyone should sell everything that begins with a ‘C’, but nothing that begins with an ‘A’.”. According to this moral theory, the moral claim “Anna should not sell Christoph`s car” would be right for English speaking Anna, but wrong for the German-speaking Christoph, because “car” for him begins with an ‘A’ (“car” in German: “Auto”).

It follows that the moral theory “Everyone should sell everything that begins with a ‘C’, but nothing that begins with an ‘A’.” cannot be the correct moral theory, because it is incompatible with moral realism.

Special relativity

Einstein’s theory of special relativity describes the relationship between space and time, and its predictions have been confirmed by many experiments (Roberts & Schleif,2007). One central statement of special relativity is that simultaneity (among other things) is relative. That means that for observers that move at different (constant) velocity, different events take place at the same time.

Figure 1: Two observers (A and C) moving relative to each other, perceive different events to happen simultaneously.

Let us take a look at the example of Anna (A) and Christoph (C) that can be seen in figure 1. Each of them is traveling at half the speed of light into the center of a square, coming from 2 adjacent sides of the square. From our perspective (in the rest inertial system of the square), both depart from their origin at the same time and arrive at their destination (the other edge of the square) at the same time. However, statements about which events take place at the same time, are not absolute and vary from inertial system to inertial system. From the perspective of Christoph, he himself does not travel with half the speed of light to the left but is at rest, and the square moves with half the speed of light to the right. Also, Anna’s trajectory is different for Christoph than for us. For him, she departs from her origin, before Christoph departs from his wall, and she arrives at her destination after Christoph reaches his destination. (Or in Christoph’s terms: before his destination reaches him.)

For Anna, the situation is the other way around. In her inertial system, she departs after Christopher but arrives before him.

It is important to understand here that neither of them is more right than the other. Neither of them has false beliefs. This might seem counter-intuitive, as Anna claims, to arrive at her destination before Christopher, and Christopher claims to arrive at his destination before Anna.

It seems as if at least one of these claims must be false. But in which order two (space-like separated) events occur, is just a matter of description. Just like the English-speaking Anna could point at a car and claim “The name of this object begins with a `C`.” and the German-speaking Christoph could point at the same car and claim “The name of this object begins with an `A`.”. At a first glance, these claims seem to be mutually exclusive, but once you understand that they use different languages to describe the same object, you realize that both can be right within the context of their own language. There is no one true language, to settle the question, what a car should be named.

There is also no preferred inertial system, out of which about the order of events might be settled. The statement that all inertial systems are equally valid and that the physical laws are the same in all of them is called the “relativity principle” and is a central claim of special relativity.

The thought experiment

In the following thought experiment, we assume both moral realism, and time discounting, and show how this leads to a contradiction. So, I am assuming that there is a correct moral theory and that this theory values events less, if they are further in the future.

Figure 2: For two observers with relative velocity (A&C), moral statements vary according to moral theories that contain time discounting.

Let us consider the situation depicted in figure 2: There is a big square.

At the middle of each side of the square is one person: Sven (S), Elsa (E), Christoph (C), and Anna (A).

At the time Christoph begins traveling with a velocity of in the direction of Sven, and Anna begins to travel with a velocity of in the direction of Elsa. In the middle of the square there is one carrot.

When the carrot arrives at Elsa or Sven, some event will be triggered, which is morally desirable according to the correct moral theory. If the correct moral theory was total hedonic utilitarianism with time discounting, this would mean for example that Elsa and Sven would be happy to receive the carrot. The moral desirability is the same whether the carrot arrives at Elsa of Sven.

Christoph and Anna arrive at the same time in the middle of the square, and only one of them can take the carrot with them. From our perspective, it makes no moral difference, who takes the carrot. If Anna takes the Carrot, Elsa would be happy to receive it, and if Christoph takes it, Sven would be just happy to receive it. And both would receive the carrot at the same time.

Let us now look at the situation from Anna’s perspective. From Anna’s Perspective, she arrives earlier at Elsa, than Christoph arrives at Sven. So, if Anna takes the carrot, the carrot would be received earlier than if Christoph would take the carrot. If the correct moral theory would not contain time discounting, it would still not matter who took the carrot because both Elsa and Sven would be equally happy to receive it. But because we assume here that the correct moral theory does contain time discounting, it is morally preferable that Anna should take the carrot, rather than Christoph. We can say that “Anna should take the carrot.” is true for Anna.

For Christoph on the other hand, he arrives at Sven before Anna arrives at Elsa. For the same reasons “Anna should take the carrot.” is true for Anna, it is false for Christoph.

Now we have the situation that a moral statement, namely “Anna should take the carrot.” is true for one person, but false for another person. But this is in contradiction with moral realism.

Since we have arrived at a contradiction, we need to discard one of our premises. Either we have to discard moral realism and say that moral statements have no truth value like factual statements, or we have to discard time discounting, and say that the correct moral theory does not value events less, just because they take place further in the future.

Possible counter-arguments

In the following, I will list possible counter-arguments, one might have, and discuss their validity.

Too absurd thought experiment

Most people concede that thought experiments can be useful to probe moral intuitions and to explore the decisions to which different moral premises might lead. But even Derek Parfit, infamous for thought experiments bordering on the absurd, recognized that there is such a thing as a too absurd thought experiment. These he called “deeply impossible” and said that in such situations “We may be unable to imagine what such a case would involve”(Parfit,1984,388).

One might reject my argument because we need to consider too absurd a thought experiment for this argument to work.

Yet, Parfit’s reason, and as far as I know the only reason, not to trust too absurd thought experiments, is that we should not trust our moral intuitions in contexts, so much different from what we ever realistically encounter. This is a good reason to distrust any argument that goes like this: “If we assume A, then it follows that we should do X in absurd scenario B. X goes against our moral intuitions. Therefore, we should reject A.”

This is not the structure of my argument. My argument goes like this: “If we assume A then it follows that we should do X in absurd scenario B. X is in logical contradiction to A. Therefore, we should reject A.” My argument does not rely on moral intuitions.

Therefore, the counter-argument that the thought experiment is too absurd, cannot be applied here.

One might still use Minkowski norm discounting

Even though the time distance between two events, and the space distance between two events is relative according to special relativity, there is a form of distance which is still the same for all observers: The Minkowski norm

If the correct moral theory would contain Minkowski norm discounting, Anna and Christoph would be indifferent to who takes the carrot with them, when they meet in the middle. But this time, Sven and Elsa come to different conclusions, who the carrot ought to go to. At the time (in Elsa’s and Sven’s inertial system), when Anna and Christoph meet at the carrot, the Minkowski norm between Elsa and the arrival of Anna at Elsa is bigger than the Minkowski norm of Elsa and Christoph arriving at Sven. So, for Elsa “Anna should take the carrot.” would be true. For Sven, it is the other way around. For him “Christoph should take the carrot.” is true.

From this, it follows that Minkowski Norm discounting is also not compatible with Moral realism.


I conclude that time discounting is not compatible with moral realism. This means that any theory that can be considered as a candidate for the correct moral theory by moral realists, must count all consequences the same, no matter how long into the future these consequences may occur.

This supports Graves and MacAskill’s claim of “Deontic strong longtermism”(Graves & MacAskill,2020,2) that the best option is mainly determined by its long term consequences in a wide class of decision situations, from a moral realist point of view. This would mean that the most important moral consideration for moral realists would be the effect of their actions on the far future.


Jeremy Bentham (1780):” An Introduction to the Principles of Morals and Legislation “, published: Oxford University Press (1907)

T.Roberts, S.Schleif (2007):” What is the experimental basis of Special Relativity?

Derek Parfit (1984):” Reasons and Persons “, published: Oxford UniversityPress (1984)

Hilary Greaves and William MacAskill (2020):” The Case for Strong Longtermism