Defining mental illness is difficult, but relatively easy compared with defining mental health. This is what I wrote in "Life at the Border":

No one is "perfect." People are human, and sometimes act irrationally. The irrationality must be the exception rather than the rule. In my opinion, mentally healthy people will show the following characteristics at least 95% of the time:

1) Try to enjoy life to the fullest.

2) Do not waste time and energy worrying.

3) Consider what others feel and have to say, but take actions according to their own convictions.

4) Do not feel distressed when others don’t like them.

5) Organize their lives functionally so that important things get done on time.

6) Can love other people easily and without fear.

7) Can find inner peace and strength when they need it.

8) Laugh a lot, and are able to find humor in most situations.

9) Look for solutions instead of complaining about the problems.

10) Act out of their own self-interest.

11) Have a strong and consistent set of values.

12) Face new ideas with an open-minded skepticism.