@HorstWurst If you have a copy of Lovecraftesque there is an ectensive essay in there about it. (If you don’t maybe @BeckyA or @rabalias might have a link to essay? (Worst case I can (with their permission) make a pdf of those pages and point you to it.))
Even Lovecraft himself wasn’t really using the term usefully - “sanity” (IIRC) has only been used in law cases - it has no clinical definition.