Thanks for the great summary. I have to say, I’m sympathetic to David’s argument here, but did get rather lost with the classification of so many different types of theory. I think you’ve made that a bit clearer.

Like you though, I’m not entirely sure about what David says on the reason to vote. I can see that there will be some collective action problems where we want to avoid free-riding by holding that all have something like a fair play obligation to contribute. It’s not obvious, however, that I have any reason to help prevent a disaster if it’s obvious that my contributions won’t do any good. Nor, I think, can we generalize to cases where there isn’t a disaster - so as long as I’m sure sufficient people will vote, I don’t see why I have to.

As for some of your points, I’m with you on 3 - we’d ordinarily say that a change can be realistic, but it doesn’t seem a problem if David wants to stipulate technical classifications. I’m not so sure about point 4 though. Surely saying something is possible is saying something like he could do it if he wanted. What I might question is whether someone really could move his arms *exactly* like someone else, but David’s example doesn’t seem to rely on exactitude - and could easily be replaced with other cases, like insulting your boss (easy but unlikely).

Thanks for the great post. I have just one quick thought on theories that David calls factually utopian. You say, and I agree, that we shouldn’t be too quick to reject theories that are factually utopian, since even if some course of action is not possible given the current facts, we might be able to one day change the facts which make that course of action impossible, and so we need to know whether we ought to aim at this result, that is, whether we should try and change the facts.

I thought it might help, at this point, to bring in the distinction Derek Parfit makes between things that are technically impossible and things that are deeply impossible (Reasons and Persons, p. 388). Things are technically impossible if they are practically impossible given current conditions, whereas things are deeply impossible if they cannot be achieved without fundamental changes to the laws of nature, including human nature.

It occurred to me that David’s account of factually utopian theories might have been meant to cover both types of impossibility, or it might only have been meant to capture the latter type. David cites, as examples of theories that are factually utopian, theories that would require people to be immortal or for there to be unlimited natural resources. These seem like things that are deeply impossible rather than technically impossible (is it possible to have unlimited natural resources…?). Anyway, David might want to say that if some theory is only technically impossible, then it is not factually utopian since our theory could require that we work to alter the facts. Theories cannot, however, require that we alter the laws of nature, and so any such theories could not be action-guiding, though maybe they could, as you suggest, still be serve some evaluative function in comparing possible worlds or states of affairs.