The authors imply (or explicitly state?) that any positive rate of pure time discounting would guarantee that strong longtermism is false (or at least that their arguments for strong longtermism wouldn’t work in that case).
But I think that this is incorrect. Specifically, I think that strong longtermism could hold despite some positive rate of pure time discounting, as long as that rate is sufficiently low.
How low that rate is depends on the size of other factors
E.g., the factor by which the value influenceable in the future is larger than that influenceable in the present
E.g., the rate at which it becomes harder to predict consequences that are further in the future.
(I’m pretty sure I’ve seen basically this point raised elsewhere, but I can’t remember where.)
The specific statements from Greaves and MacAskill I think I disagree with are
In particular, [an assumption we make] rules out a positive rate of pure time preference. Such a positive rate would mean that we should intrinsically prefer a good thing to come at an earlier time rather than a later time. If we endorsed this idea, our argument would not get off the ground.
To see this, suppose that future well-being is discounted at a modest but significant positive rate – say, 1% per annum. Consider a simplified model in which the future certainly contains some constant number of people throughout the whole of an infinitely long future, and assume for simplicity that lifetime well-being is simply the time-integral of momentary well-being. Suppose further that average momentary well-being (averaged, that is, across people at a time) is constant in time. Then, with a well-being discount rate of 1% per annum, the amount of discounted well-being even in the whole of the infinite future from 100 years onwards is only about one third of the amount of discounted well-being in the next 100 years. While this calculation concerns total well-being rather than differences one could make to well-being, similar considerations will apply to the latter. [emphasis added]
I assume they’re right that, given that particular simplified model, “the amount of discounted well-being even in the whole of the infinite future from 100 years onwards is only about one third of the amount of discounted well-being in the next 100 years”
(I say “I assume” just because I haven’t checked the math myself)
But as far as I can tell, it’s easy to specify alternative (and plausible) models in which strong longtermism would remain true despite some rate of pure time discounting
E.g., we could simply tweak their simple model to include that well-being per year expands at a rate that’s above 1% (i.e., faster than the discount rate), either indefinitely or just for a large but finite length of time (e.g., 10,000 years)
This could result from population growth or an increase in well-being per person.
If we do this, then even after account for pure time discounting, the total discounted well-being per year is still growing over that period.
This can allow the far future to contain far more value than the present, and can thus allow strong longtermism to be true.
(Of course, there are also other things that that particular simple model doesn’t consider, and which could also affect the case for strong longtermism, such as the predictability of far future impacts. But there are still possible numbers that would mean strong longtermism could hold despite pure time discounting.)
The authors imply (or explicitly state?) that any positive rate of pure time discounting would guarantee that strong longtermism is false (or at least that their arguments for strong longtermism wouldn’t work in that case).
But I think that this is incorrect. Specifically, I think that strong longtermism could hold despite some positive rate of pure time discounting, as long as that rate is sufficiently low.
How low that rate is depends on the size of other factors
E.g., the factor by which the value influenceable in the future is larger than that influenceable in the present
E.g., the rate at which it becomes harder to predict consequences that are further in the future.
(I’m pretty sure I’ve seen basically this point raised elsewhere, but I can’t remember where.)
The specific statements from Greaves and MacAskill I think I disagree with are
I assume they’re right that, given that particular simplified model, “the amount of discounted well-being even in the whole of the infinite future from 100 years onwards is only about one third of the amount of discounted well-being in the next 100 years”
(I say “I assume” just because I haven’t checked the math myself)
But as far as I can tell, it’s easy to specify alternative (and plausible) models in which strong longtermism would remain true despite some rate of pure time discounting
E.g., we could simply tweak their simple model to include that well-being per year expands at a rate that’s above 1% (i.e., faster than the discount rate), either indefinitely or just for a large but finite length of time (e.g., 10,000 years)
This could result from population growth or an increase in well-being per person.
If we do this, then even after account for pure time discounting, the total discounted well-being per year is still growing over that period.
This can allow the far future to contain far more value than the present, and can thus allow strong longtermism to be true.
(Of course, there are also other things that that particular simple model doesn’t consider, and which could also affect the case for strong longtermism, such as the predictability of far future impacts. But there are still possible numbers that would mean strong longtermism could hold despite pure time discounting.)