What is sanity?

I'm working on a story titled "Down Among the Sane Men." It will include several definitions of sanity, which will conflict.

And will have problems.

Very well adjusted to one's society and culture -- what happens when these change? And does this mean adjusted to the norms the society and culture proclaim, or the way things actually work?

Thinking the same way as the majority -- suppose majority opinion shifts drastically?

Having a really healthy mind -- as judged by what standard?