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?
There are always differing opinions in existence. You have yours, I have mine. Which are correct, which are false? The real answer is that they’re all correct AND incorrect at the same time, because everyone is subject to their own personal experiences and points of view.
From an existential understanding (perhaps spiritual), the universe manifests in all forms, in all times, in all spaces, simply to experience existence as throughly as possible to exhaust all possibilities through various combinations and permutations. If one could equate this to “God” or some manifestation of the divine reality, then regardless of bias, they are all necessary for existence.
So, it’s very simple. Whether someone is right or wrong holds no meaning, only that you accept it, because whether or not you agree, it still exists and therefore is fundamental to the entirety of existence.
At least, that’s my opinion. You may think or believe differently, and that’s okay.