I find those afraid or unwilling to question their beliefs, not least of all unquestioning belief in their own sanity, highly suspect. An unquestioning certitude regarding one's own sanity is not a safe presumption to make and isolates oneself from the means to monitor and assess the validity of their beliefs and correlation between what they believe and the reality to which their beliefs should correspond.
The belief that sanity is an either/or state of being and that one should therefore presume their own sanity until proven otherwise only opens the door and lays out the welcome mat to a host of no less and ever increasing unfounded presumptions that are inevitably and invariably certain to follow.
No one is immune to or completely devoid of some degree of mental error that left unchecked will leave them floundering in a river of denial flowing ultimately into a sea of delusion.
Doubting one's own mental infallibility is the first step to assessing and evaluating what stage of insanity each one of us has reached and discovering the means to deal with and be responsible for our own mental health before we succumb to the more generally accepted and widely acknowledged classic definition and full blown manifestation of the term. Responsibility for monitoring and maintaining our own mental and intellectual health is an inescapable and unavoidable reality.
Perfect sanity, apart from any quantifiable and qualifiable standard is a deceptive delusion. Those who refuse to question and measure the degree of their own insanity are typically the last to know.