I recently read Greaves & MacAskill’s working paper “The case for strong longtermism” for a book/journal club, and noted some reactions to the paper. I’m making this post to share slightly-neatened-up versions of those reactions, and also to provide a space for other people to share their own reactions. I’ll split my thoughts into separate comments, partly so it’s easier for people to reply to specific points.
I thought the paper outlined what (strong) longtermism is claiming - and many potential arguments for or against it - more precisely, thoroughly, and clearly than anything else I’ve read on the subject. As such, it’s now one of the two main papers I’d typically recommend to someone who wanted to learn about longtermism from a philosophical perspective (as opposed to learning about what one’s priorities should be, given longtermism). (The other paper I’d typically recommend is Tarsney’s “The epistemic challenge to longtermism”.)
So if you haven’t read the paper yet, you should probably do that before / instead of reading my thoughts on it.
But despite me thinking the paper was a very useful contribution, my comments will mostly focus on what I see as possible flaws with the paper - some minor, some potentially substantive.
Here’s the paper’s abstract:
Let strong longtermism be the thesis that in a wide class of decision situations, the option that is ex ante best is contained in a fairly small subset of options whose ex ante effects on the very long-run future are best. If this thesis is correct, it suggests that for decision purposes, we can often simply ignore shorter-run effects: the primary determinant of how good an option is (ex ante) is how good its effects on the very long run are. This paper sets out an argument for strong longtermism. We argue that the case for this thesis is quite robust to plausible variations in various normative assumptions, including relating to population ethics, interpersonal aggregation and decision theory. We also suggest that while strong longtermism as defined above is a purely axiological thesis, a corresponding deontic thesis plausibly follows, even by non-consequentialist lights.
 There is already a linkpost to this paper on the Forum, but that was posted in a way that meant it never spent time on the front page, so there wasn't a time when people could comment and feel confident that people would see those comments.
There's also the post Possible misconceptions about (strong) longtermism, which I think is good, but which serves a somewhat different role.
 Other relevant things I’ve read include, for example, Bostrom’s 2013 paper on existential risk and Ord’s The Precipice. The key difference is not that those works are lower quality but rather that they had a different (and also important!) focus and goal.
Note that I haven’t read Beckstead’s thesis, and I’ve heard that that was (or perhaps is) the best work on this. Also, Tarsney’s “The epistemic challenge to longtermism” tackles a somewhat similar goal similarly well to Greaves and MacAskill.
This post does not necessarily represent the views of any of my employers.