Doug, my point regarding teaching is that the definition being offered previously yields absurd results. Teaching is not lying, but it is, in the strictest sense, a set of statements known by the speaker to be untrue. That’s why I claim that our definition of lying needs to be more complicated.
Well, OK, but I think the real problem is some elision here of a couple of concepts of lying. The simplest one (the lie?) is that a lie is a statement made that is known to be untrue. That’s fine as far as it goes, but as you note, we may not be precisely lying when saying things we know to be false—namely, when we say things that may be strictly false but are approximately true, when done in a context where that approximation is good enough to get the job done.
E.g., you don’t need to know QM to throw a ball into a bucket, and anyone who tells you you do is lying to you.
Re. the case of stagecraft, perhaps we could amend the concept to something like:
(1) X tells a lie iff X sincerely makes a statement he knows to be untrue.
(Where to make a statement sincerely is to make it in a context where it is taken as purportedly true; e.g., not on a stage).
I seem to remember that Harry Frankfurt gave a definition of lying in one of his small books, the ones that truthaddict mentioned. They are absolutely brilliant. If he did so, I’m sure it’s better than the one I just gave.