A rolling stone

I was watching the great 1975 movie One Flew Over the Cuckoo's Nest, and there is a scene in which some shrinks are trying to decides whether Jack Nicholson is crazy. One shrink asks:

Have you ever heard the old saying, A rolling stone gathers no moss? ... Does that mean something to you?

Jack responds:

It's the same as don't wash your dirty underwear in public. ... It's hard for something to grow on something that's moving.

The question is one that shrinks really use to test for mental illness. The idea is that schizophrenics will not grasp the abstract thinking that is required to understand a simple proverb.

The advantage of this particular proverb is that no one knows what it really means. It dates back to some Roman slave 2000 years ago. The rolling stone appears to be some sort of metaphor for someone who moves around a lot, but it is not clear whether growing moss is supposed to be good or bad. Jack's answer is brilliant because he not only answers the question, but also throws back his own abstract and meaningless analogy.