Sanity is the " baseline" of "mental health" that is established by means of observing the majority of people's pattern of thinking,priorities, perspectives, et cetera; such that people that deviate from the norm can be labelled as defective or can be repudiated and their existence denied by the majority of the population when " fun" can be made of someone else's existence or it is convenient to do so. For example when a person-let's call them A- uses a negative stereotype against someone else-B-, they may try to get out of the ethical implications by viewing those of a group that do not follow the negative stereotype as being nonexistent or defective even when simple deduction, reasoning, empathy,or understanding would have rendered the stereotype unethical and false as a sweeping all encompassing generalisation of a large group of people; in other words so that some one can be stigmatised
if you want to define someone as sane or insane, you'd look at what they were doing in terms of how much practical sense it made, and also how it compared to what others in similar situations are doing. the more an act deviates from what is generally regarded to be practical, or from what the majority would do, the more it is labeled insane. but since what is regarded as practical isn't always practical, and what the majority considers appropriate isn't always appropriate, the terms sanity and insanity would inherently be somewhat subjective.
ACK! My own words Res? Don't you know quotes are my lifeblood?
Okay. Here it goes.
A sane person knows how insane they are. They look in the mirror once they've done something crazy and go "Yeah, -- that was fucked up." On the other hand, insane people think everything they do is perfectly sane. It's like the serial killer who goes around hacking up prostitutes; they don't think it's crazy, it's like they're doing humanity a service. The rest of us are left to think WTF?