I meant to post about this a while back, but back I was home in the scenic New Jersey suburbs for Thanksgiving, I caught an episode of Law and Order that I ended up puzzling over for a bit. As it was blessed with neither competent acting nor a particularly compelling script, I’ll just stick to the bare-bones description: A man is on trial for shooting a serial murderer who had escaped and gone on a rampage, killing (among many others) the defendant’s young daughter. Upon the killers recapture, the DAs had done their best to find their way around a precedent that prevents the death penalty from being applied, but were unsuccessful. The defendant is basing his defense on the claim that he went (temporarily?) insane when he heard this news, and the trial portion of the show devotes a lot of energy to the question of whether he “knew the difference between right and wrong” and, in particular, “knew what he did was wrong” at the time of the killing.

What makes this interesting—and throws into relief the oddness of this phrasing—is that almost nobody involved does actually think that killing the victim was “wrong” as such. At most, they think it was wrong in the absence of proper legal procedure, and plenty of them don’t even seem to think that. Sam Waterston’s character, hammering again and again on whether the defendant “knew” he’d acted wrongly, as though it were some straightforward fact, like “knowing” you’re not Napoleon, starts to sound naive to the point of derangement himself. Because it’s clear the defendant doesn’t “know” he acted wrongly: He believes he acted correctly. But this moral disagreement doesn’t make him insane—given that people have plenty of disagreements about what’s right or wrong as a matter of course, it would be strange if it did. (For one, plenty of drug users could sincerely claim not to “know” they’d done anything “wrong”…)

The obvious response is to say that’s all beside the point, because what’s really at issue, despite all this talk of “right” and “wrong,” is whether the defendant understood he was breaking the law. But that’s a little weird too, as we normally go by the principle that “ignorance of the law is no excuse.” That’s just as well, since I’m guessing it’s actually pretty common that people who fly off the handle and throw a punch or break something in a fit of anger aren’t consciously aware they’re committing a crime when they do it. (Though it may only be a few seconds before they stop and think: “Oh, hell, what did I just do?”) I’m thinking “temper tantrum,” or the ordinary “crime of passion,” is probably too low a bar for a valid insanity defense. And perhaps more the point, it’s pretty easy to imagine a genuinely insane person nevertheless being fully cognizant that what they’re doing is a violation of the law. They might just be in the grip of an “irresistible impulse” (which I think is the other legal standard that’s sometimes used) or believe that they were being commanded by God, or that the person they killed was really a demon, or what have you.

This points, I think, to a pretty intuitive clarification of what kind of failure to “know what you did was wrong” counts: On the one hand, again, you might have that plain old “irresistible impulse.” On the other, you might have some kind of defect in the way a person forms factual beliefs, severe enough to be distinct from garden variety epistemic carelessness, and (crucially) yielding a belief that, if true, would make the person justified in thinking his action wasn’t wrong, so you eliminate mere normative disagreement as an exculpating factor. Presumably, something like this standard is what lawyers and judges and juries are actually using; I just found the usual locution weird enough that it seemed worth noting explicitly.