I've heard or read the following enough times now to consider that it's becoming generally accepted:
"The definition of insanity is doing the same thing over and over, and expecting a different result each time."
I'm not sure why this is catching on. You clearly can get a different result each time you do the same thing. (For example, you can get better at it.)
And even if the repeated action is futile and you're foolish enough to repeat it- is that really insanity? What about Charlie Brown and the football? Clearly, Charlie Brown has a problem, and he needs to wise up, get some self respect, and stop listening to Lucy. But is he actually insane for expecting her to hold the football properly for him? Is this the definition of insanity?
Part of the trouble is that a word like "insanity" has no good definition (although we can clearly do better than that one). Like the word "weird," it's used as a desperation device; if you really want to describe something, these words are too subjective and too glib to even bother with. If you find that, nevertheless, you're quite attached to words like "weird" and "insane," a movie like Mondo New York can really help you deconstruct, question, and perhaps dispense with them.
In Mondo New York, which Jamie and I watched on a creaky old VHS tape with our neighbors, we meet a parade of New York City artists of the 80's, and among these is Joe Coleman, who obliges us with the signature move of his early days of performance art: he bites the heads off two live mice that he holds in his hands. He also shouts a kind of obscenity-laced free verse poetry, writhes on the stage with his feet in the air, explodes a package of fireworks strapped to his chest, and runs out of the theater screaming.
Coleman proves to me that there is no such thing as insanity- at least, there isn't a way to categorize people as sane or insane. He behaves in a way that many would describe as "insane," but if you read more about his work and his ideas (his own website is a great place to start), he's trying to get in touch with parts of the human psyche that are inside all of us. As if to acknowledge this, the film later parallels Coleman's performance with a voodoo ceremony in which a priest bites through the neck of a live chicken and, rather horribly, takes blood from its twitching spinal column to mark the foreheads of the worshipers in attendance. There are good reasons to object to animal sacrifice, but it would be lazy to call it "insane," since, as I think Mondo New York shows us (intentionally or not), it's a bit more fundamental to who we are than our daily lives would suggest. Perhaps "insanity" is merely that which is naturally occurring, but so rarely manifested that we find it unpleasant (a greater degree of "weirdness").