Zimmerman (1996: 118) says that you shouldn't try to represent conditional obligation in terms of a material conditional where the 'ought' operator takes wide-scope. At first I thought I got it, but now I'm not so sure.
(1) You should vote Obama but if you don't you shouldn't vote at all.
Let A: Vote Obama.
Let B: Vote for no one.
Letting the arrow stand for the material conditional, the idea is that we model conditional obligation (e.g., (1)) as follows:
(2) O(~A --> B)
Here's the problem with that. Suppose O(A). O(A) entails O(AvB), which entails O(~A-->B).
Now, let C: Vote for Romney. Suppose O(A). O(A) entails O(AvC), which entails:
At first I thought that his problem with (2) was that (2) was too weak and that there's got to be more to conditional obligation than (2). That still seems right, but now I'm worried. I could be wrong, but doesn't (1) entail?
Doesn't (4) entail (2) and (3)? Doesn't it seem that (1) is incompatible with (3)?
I've been traveling all afternoon and evening, reading Zimmerman on the plane and then driving back to Austin from San Antonio. It is now 1:14 AM and I've just stepped in the door (can't sleep), so there's a non-zero chance that I'm just missing something obvious, but I can't tell whether the problem is that (2) is false or that it's just too weak. What do we have to give up to block the inference from (4) to (2)? He doesn't deny that O(A) entails O(AvB). (His solution to Ross' paradox is the one that I'd offer and it doesn't require denying that inference.)
Maybe the idea is that (1) is compatible with (2) and (3)? Maybe that's right, and maybe the problem is just that (2) is too weak to capture the conditional obligation stated in (1). (AvB) and (AvC) are logically equivalent to (~A --> B) and (~A --> C) respectively and to ~(~A & ~B) and ~(~A & ~C). Just as there's no deontically superior world accessible to this one where you neither A nor B on the assumption that the best accessible world is an A-world, there's no deontically superior world accessible to this one where you neither A nor C on the assumption that the best accssible world is an A-world.
So, maybe the idea is that (1)-(4) are all true and that's consistent with the denial of:
(5) You should vote Obama but if you don't you should vote for Romney.
Why not? If there's more to (1) than (2), then there's more to (5) than (3) and that extra stuff is what we can use to capture the intuition that there's something wrong with (3) that really is the intuition that (5) can't be true if (1) is.