I'm stuck in grading and can't get finish off a post I'm pretty excited about but am not ready to post yet, so here's something in the meantime from a while back: lying by imperatives. I saved this and never got around to posting it, probably because I had something to say about it, but I don't have the time to read it carefully again and see what that might have been. I suspect it had something to do with the ethics of lying. Maybe I'll post an update tomorrow if I come up with anything once I'm through my last class of the semester.
Update: I think I remember what I might have wanted to say, but I'm not entirely sure this is exactly the point I had in mind. The lying guard says. "Don't bite me" in place of the usual "Bite me", as if the imperative has a truth value, which it doesn't if you're actually going to be precise about the semantics. But we understand what's going on here when someone commands the opposite of you want to do as an instance of being a complete liar, because there's something truth-related about imperatives. Imperatives often assume propositional content that does have a truth value, and they communicate propositional content that does have a truth value.
Another example would be if I tell you to stop poking me. That's an imperative, but the act of saying the imperative, even though it has no semantic truth value, does pragmatically convey the information that I think you're poking me and the information that I want you to stop. So whether a speech act counts as lying might depend not just on whether you deliberately communicate false semantic content. Deception can occur when you communicate false information pragmatically, and the cartoonist seems to understand that in having a lying guard present the opposite imperative of what the guard wants the person to do. It doesn't even have to be a speech act. It can simply be an act. You try to communicate that you're home by leaving the lights on when you're out. If lying is a deliberate attempt to deceive by communicating false information, then you don't need to say something that's semantically false to lie. You can say something semantically true but misleading, expecting to communicate an additional falsehood. You can say nothing at all but by your actions intentionally communicate something false.
This has implications for the moral issues involving lying. Some people hold that lying is always wrong no matter what. I know people who hold such a view but then stick to the letter of the law about not expressing falsehoods semantically while being fine with deliberately deceiving people pragmatically. I would suggest that what's bad about lying is still present in such cases, whether we call it a lie or not (but I would prefer to call it at least a lie of sorts). If any case of deception is ok, why shouldn't it be ok to lie semantically in such cases?
[For more on the ethical issues, see my earlier post on the ethics of lying.]