I think it’s a fun question. I’d argue that sanity doesn’t exist, as the concept of sanity is constructed entirely of opinions–if a concept is made up of opinions, then the concept itself is an opinion, and is therefore entirely hypothetical.
How do you know you’re in your right mind?
Sanity is measured by the coherencecy of our natural experience and the simplicity by which it demonstrates that. The simplest measure of insanity is psychosis and so sanity is alignment of experience with the concepts of its objects.