Then you don't like the world "illness" in general. The same holds for physiological health: none is completly bacteria-free, has perfect levels of vitamins, etc. However, we need to operate in our judgements with ideals (healthy; sane; circle and all other mathematical entities, etc.): reaching for them, although never succeeding; that's perfectly okay.

Oh, and we do have some quite good (attempts at) theories of what "sanity" is all about.