Suppose Smith and Jones are shooting at each other each in the hopes of killing the other. Let's also suppose that Smith's shooting is justified, say, on grounds of self-defense. Is it possible that Jones' shooting is justified as well?
Most will grant that it is possible for S to have _a_ justification that conflicts with _a_ justification J has, but that seems consistent with saying that it is possible only that S's actions are justified or that J's are. There can be conflicts among the pro tanto reasons each agent has for acting, but just as those conflicts will resolve themselves in determining what S ought to do overall they will resolve themselves in determining whether S ought to shoot J, J ought to shoot S, or ensuring that neither ought to shoot the other. The conflicts will not surface at the level of all-thins considered oughts, say.
To give the thesis a name, a name I believe I'm lifting from George Fletcher, let's say the incompatibility thesis is the thesis that situations cannot arise where S's actions are justified, J's actions are justified, but just as S's success in acting requires preventing J from acting, J's success in acting requires preventing S's success in acting. Let's say the coherence thesis is the thesis that situations cannot arise where S all-things considered ought to perform an action, J all-things considered ought to perform an action, but the situation is such that S's success comes at the expense of J's success (and vice-versa).
I'm somewhat inclined to reject both the incompatibility thesis and the coherence thesis. Let me begin by noting something I think is interesting.
Subjectivism and OIC
It seems reasonably clear that the denial of the coherence thesis commits you to denying that all-things considered ought implies can. To deny the coherence thesis is to assert that there are situations in which it cannot be that S and J both succeed in doing what they all-things considered ought to do. Now, suppose you adopt a view on which what ought to be done is a function of one's perspective on the situation rather than a function of the facts as they are. It would seem that on such a view, depending on how we describe S's perspective and J's perspective, it might seem best given S's perspective for S to X and best from J's perspective for J to Y even though the situation is such that it is impossible for S to X while J Y's. If this truly is a case where the subjectivist would deny the coherence thesis and the denial of the coherence thesis comes with denying ought implies can, then we can see what is wrong with using ought implies can to motivate subjectivist or psychologized approaches to deontic notions such as obligation.
Maybe my intuitions are just so thoroughly corrupted that they oughtn't be trusted, but it seems to me that there are roughly a gazillion counterexamples to the incompatibility thesis. Here's one. S and J are lovers. S likes to wake before J so that S can make coffee. J likes to wake before S so that J can make tea. If they wake at just the same time, there's an awkwardness in determining whether to make coffee or tea and typically they end up having neither. It is clear that J's making the coffee is justified as is S's making the tea, but neither could carry out their justified actions if the other were to try to carry out the actions they were justified in performing.
It might seem harder to construct counterexamples to the coherence thesis working with an objectivist approach to justifications and obligations, but the following strikes me as being pretty compelling. J is dropping bombs in a just war and we might suppose that he is not only permitted to drop the bombs but obligated to do so given the importance of this particular campaign. S is a non-combatant who will be killed as a side-effect if J's bombs drop. If this were merely a case of S's defending himself, we might think that he would be permitted to either take arms against J or do nothing. Let us suppose, however, that S's family will also be killed in the campaign. It is not obviously wrong (to me, at any rate) to say that S ought all-things considered man an unmanned anti-aircraft gun and fire on J's plane in order to prevent J from dropping the bombs.
I cannot see any reason to reject this description of the case: S ought to drop the bombs the dropping of which would prevent J from preventing S from dropping the bombs; J ought to fire on S's plane in such a way as to prevent J from dropping the bombs, it cannot be that both S and J do what they ought to do.
Someone might say that denying the coherence thesis prevents morality from playing the role it ought insofar as it prevents morality from resolving conflicts.
This objection strikes me as rather weak. It still gives the agents some advice in this situation (take arms against the incoming planes if you must, but under no circumstances must you turn turn those guns on other non-combatants) and can still give advice in other situations. I cannot think of any reason to think that morality has to give more advice than this.
I worry that this objection might also prove too much. Problems with the incompatibility thesis arise when we have situations where S is permitted/justified in X-ing while J is permitted/justified in Y-ing while it is impossible that both S and J get their way. Intuitively it seems we just let J and S sort this out and neither acts wrongly unless one acts against a side-constraint in the way that they promote their ends. (If J just wakes up first, fine. If J wakes up first because J drugged S that's a different matter even if that is the only way for J to wake up first.) However, if we thought that morality had to guide us to unique outcomes in conflict situations, should that not lead us to assert the incompatibility thesis and insist that either S is not really justified in S's actions, J is not justified in J's actions, or both? I don't think you can derive the falsity of the coherence thesis from the falsity of the incompatibility thesis, but as the incompatibility thesis seems not particularly plausible in light of examples, theoretical justifications of coherence that would thereby establish the incompatibility thesis are suspect.
So, what are the other principled objections to denying the coherence thesis?