Some would say it's the wrong thing to divide functionality into categories.
But, don't go crazy, it might be possible!
It depends on how desperate you are!
1. The experiencing brain model: the experience of the brain might be the basis for functionality.
2. The contribution model: people who contribute functional things might be functional people.
3. The romance with sanity model: people who fall in love with sanity might be the most sane.
4. The rational object model: people who have a rational object might be the rational ones.
International Society for the Philosophy of Architecture » Ethics and Aesthetics of Architecture and the Environment