## The Grim Reaper Paradox

There is a long tradition, in philosophy, of employing paradoxical thought experiments in order to show that our understanding of some subject is either incomplete or incorrect. Quite famously, the paradoxes of Zeno of Elea puzzled philosophers and mathematicians for millennia. These enigmas can be, at once, immensely entertaining and thoroughly maddening to contemplate.

About a year ago, I was introduced to one such thought experiment which I had not previously encountered. It is known as the Grim Reaper Paradox, and the version with which I will interact today is presented by philosopher Alexander Pruss. The thought experiment proceeds as follows:

Fred is sitting in a room at 8:00 am. There exists an infinite number of Grim Reapers along with Fred, each of which is currently dormant. When any individual Grim Reaper becomes activated, if Fred is still alive, then that Reaper will instantaneously kill Fred; however, if Fred is not alive, the Reaper will return to a dormant state and continue to do nothing. Each of the Grim Reapers is timed to activate at a specific time after 8:00 am. The last Reaper will activate at 9:00 am. The second to last activates at 8:30 am. The third from last at 8:15 am. In general, the *n*th from last Grim Reaper will activate after hours have passed.

Now, we are guaranteed that Fred will not survive past 9:00 am. After all, if he is alive at 9:00 am, then the last Grim Reaper will activate and kill him. However, he can’t have lasted that long, either, since the previous Grim Reaper would have killed him if he had survived until it activated. In fact, we can generalize this: the *n*th from last Grim Reaper cannot have killed Fred, because if he had survived until hours after 8:00 am, then the *(n+1)*st from last Grim Reaper would have killed him.

Therefore, we see that Fred cannot survive until 9:00 am, and yet we have also shown (by mathematical induction) that none of the Grim Reapers can have been the one which killed Fred. Thus, we have come to a paradox.

One of the fun things about a paradox is that it often illustrates that one or more of our assumptions in defining the problem must have been incorrect. Often, the resolution of a paradox comes by abandoning one of the assumptions on which the problem was founded, even if that assumption feels entirely intuitive. In the case of the Grim Reaper paradox, Pruss finds that there are two assumptions whose abandonment could lead to a resolution of the paradox (emphasis added):

So either there are only finitely moments of time between 8 and 9 am, ornocombination of Grim Reaper alarm clock settings is possible. In the latter case, it basically follows that it’s just impossible to have infinitely many Grim Reapers, whether their wakeup times are arranged so as to result in a paradox or not. Sowhycan’t there be infinitely many Grim Reapers? It seems that the only reason to suppose there can’t be infinitely many Grim Reapers, even in cases where no paradox is generated, is if one thinksthere can’t be an actual infinity of objects in existence.

The idea that time contains some finite number of total moments (where a “moment” is the smallest possible subdivision of time) is known in philosophy and in physics as Discrete Time. If time is discrete, then the setup for our paradox cannot actually occur, since there would not be an infinite number of subdivisions of the hour between 8:00 am and 9:00 am. Re-formulating the problem to fit with the notion of Discrete Time leads to an obvious answer: the Grim Reaper timed to activate in the first moment after 8:00 am is the one which kills Fred. If time is Continuous (the opposite of Discrete; that is, infinitely divisible) then there is no smallest possible subdivision of time, and therefore there is no first moment after 8:00 am, leading to our paradox.

The second assumption which Pruss illustrates is pretty much just a generalization of the first. Rather than limiting our scope to time, the second assumption is that *anything* can be actually infinite. Obviously, if there cannot be an infinite number of any object, then the setup for our Grim Reaper problem is in error, as there can be neither an infinite number of Grim Reapers nor an infinite number of moments for those Grim Reapers to occupy. While this is a natural extension of the first assumption, I don’t think that there is any warrant to reject the possibility of any and all actual infinites, from this paradox alone. Since the more narrow case of Discrete Time resolves the paradox comfortably, one would need some other method to show that infinites are impossible more generally.

Though Alexander Pruss discusses this Grim Reaper Paradox in a few of his other blog posts, I have not seen him discuss any other assumptions which might underly the problem. He seems to have focused upon these as being the prime constituents. However, it occurs to me that the problem includes another assumption, which is a bit more subtle. The Grim Reaper Paradox, as formulated, seems to presume the Tensed Theory of Time. I have discussed, elsewhere, the reasons that I believe the Tensed Theory of Time does not hold, so I’ll simply focus here on how Tenseless Time resolves the Grim Reaper Paradox.

On Tensed Time, the future is not yet actual, and actions in the present are what give shape and form to the reality of the future. As such, the actions of each individual future Grim Reaper, in our paradox, can be contingent upon the actions of the Reapers which precede them. However, this is not the case on Tenseless Time. If we look at the problem from the notion of Tenseless Time, then it is not possible that a future Reaper’s action is only potential and contingent upon the Fred’s state at the moment of activation. Whatever action is performed by any individual Reaper is already actual and cannot be altered by the previous moments of time. At 8:00 am, before any Reapers activate, Fred’s state at any given time between 8:00 am and 9:00 am is set. It is not dependent upon some potential, but not yet actual, future action as no such thing can exist.

Given that I thoroughly reject the Tensed Theory of Time, and given that I see no other reason why we should reject actual infinities, I am inclined to believe that the Grim Reaper Paradox’s assumption of Tensed Time is incorrect.

I don’t have much faith in the idea of tenseless time either.

How do you mean? Do you prefer Tensed Time? Or are you saying that you prefer something more along the lines of Julian Barbour’s entirely timeless model?

I am not sure I understand your tenseless solution here. Sure in tenseless time, the events are fixed, but how a Grim Reaper would react in each time would still be dependent upon what happens earlier. And if that is the case, then there is still the question of when, exactly, Fred gets killed and by which Grim Reaper.

Causality, on Tenseless Time, is

descriptiveand notprescriptive. Whether or not any individual Reaper kills the subject is set and unchangeable. As such, the ontology of any individual Reaper’s action is not dynamic.In this case, the Paradox simply tells us that it does not seem to be possible to arrange an infinite set with the causal relationship which the thought experiment demands.

So are you saying that such a scenario is impossible to begin with?

Yes. It would be like trying to set up a thought experiment with a triangle that has two internal right angles, or trying to discuss a negative number greater than 2.

Thanks for the response. In that case I am still not sure where tenselessness fits into this. A tensed theorist can just as well say something along the same lines: you can’t define an infinite sequence of grim reapers because it would lead to a contradiction. I am not saying that it is wrong to say that (the response sounds similar to Yablo’s in fact), but just that it doesn’t seem to have anything to do with tense.

Alexander Pruss actually mentions this possibility, in the original article to which I was replying. He says that one solution to the problem might be that this particular configuration of infinite Grim Reapers is impossible. However, he saw no reason why this particular configuration should be impossible while other infinite configurations should be possible.

Tenseless Time makes the setup of the paradox impossible, as the setup requires that the ontology of any particular Grim Reaper’s action is not set until the time of that Reaper’s activation. The thought experiment is dependent upon the concept of temporal becoming. If this concept is invalid, the whole thought experiment crashes.

For Yablo, the reason why something like the Grim Reaper paradox is impossible is because it is setup with an inconsistent set of rules. We simply can’t set up a system of Grim Reapers in that way, and expect them all to work as intended. It is like trying to program a machine to produce a triangle with four sides and expecting it to actually do so. The logic just doesn’t allow for it.

As I see it, the setup of the paradox doesn’t require temporal becoming at all. It only requires that the Grim Reaper’s actions at a particular time be dependent upon what happens at earlier times. This still makes sense under tenseless time as in tensed time, so I’m afraid I don’t see how temporal becoming is important here.

For anything to kill Fred, it must perform physical work on his physical body, so the scenario seems to fall foul of both quantum mechanics (the uncertainty principle) and relativity (that much energy concentrated around Fred collapses into a black hole, and, as on the Orient Express, they all did it.) There’s no need to invoke speculation about the nature of time.

I think you are taking the thought experiment a little too literally. The idea of “killing Fred” is simply a stand-in concept for some general change in state over time. This need not necessarily be matter or energy changing, in which case neither quantum mechanics nor conservation of energy need come into play.

However, given this revelation, it becomes clear that the Grim Reaper Paradox is really no different than Zeno’s Dichotomy Paradox, except for the fact that we are discussing a temporal dimension of measure rather than a spatial one.