John Gardner gave an interesting talk yesterday, arguing that excuses are not merely a means of avoiding responsibility (for an act), but also a way to claim responsibility (as a moral agent).
Compare other cases, e.g. insanity pleas, where one abdicates responsibility entirely. We hold each other to normative standards only insofar as we see each other as moral agents, capable of responding appropriately to reasons. But the insane are not even within the space of reasons. They are no longer considered persons at all. They thus escape legal responsibility, because it makes no more sense to hold them accountable than to hold a wild animal to account.
It is deeply shameful to be regarded as a non-agent, however. Some defendants have therefore sought to portray themselves as reasonable people, even though they admit they acted unjustifiably. This may at first seem paradoxical. Instead of offering either justifications or an abdication of agential responsibility, they try a third way: a reasonable excuse. A provocation defence, for example, might seek to establish that the defendant's emotional response (rage/anger) was a reasonable reaction to the situation. This reasonable anger then led them to act unreasonably -- killing the victim, say. But it is not as though their actions here are totally incomprehensible, such that we must regard them as a non-person, a mere force of nature. Rather, it is the response any reasonable person would have had to the situation. It is a kind of rational irrationality, or blameless wrongdoing.
I agree with Gardner that this 'third way' makes conceptual sense. (It's a further question whether the law should allow it, of course.) Indeed, it's a familiar point for consequentialists that a good character might on occasion lead one to perform bad actions. So we can make sense of the intransitivity of reasonableness if we say that a reasonable emotional response (or disposition) is one that will tend, in general, to lead to better (more reasonable) actions. This is clearly compatible with the disposition leading one astray in particular circumstances.
(I should note that Gardner wasn't entirely satisfied by this suggestion; he thinks there are some cases where it won't suffice. But I'm not sure what those cases are.)
A practical upshot: we may be able to determine what excuses should be allowed as legal defences, depending on which dispositions we wish to encourage in the general population. For example, there is nothing to be said for encouraging jealousy, so finding your lover in bed with another should not be considered a legitimate 'provocation' to murder. But perhaps it is good to feel righteous anger in response to domestic violence. So a battered spouse might have a legitimate excuse for perpetrating their revenge (even in cases where they lack the full-blown justification of self-defence). Food for thought.