NOT ENTIRELY SANE: Our sanity depends upon the regular support of insanity. Why, you may occasionally wonder, do we act as we do, rarely bothering to consider the essential nature of so many of our activities, but mostly pursuing them as though blinded to their consequences, unaware of their 'actualities', of how strange, diverse, and persistent they usually are? Clearly, because we are insane as well as sane, because it is natural for us to exploit our insanity in the interests of our sanity, our unconscious mind in the interests of our conscious mind. How on earth could we dare to call ourselves 'sane' in the first place, without its antithesis to support us and grant our sanity a reliable foundation? How could there possibly be any sanity in any of us, without the aid of its opposite? A 'sane man' per se can never exist.
Why, then, do we classify certain people as insane if, to a certain extent, we are all mad? Simply because we are largely ignorant of the matter? Possibly. But, more probably, because we habitually associate insanity with notions of incompatibility, irrelevance, superfluity, extreme eccentricity, unrelatedness, ostracism, delusions of grandeur, etc. A person who talks to himself is generally considered mad because custom and common sense normally prohibit us from following suit, since it would make us conspicuously anomalous in a world where most people talk to others. When a man persistently talks to himself in public places he not only draws attention to himself, whether compassionately or critically, but he makes it difficult for other people to communicate with him. Thus he is regarded as a madman for having employed his sanity/insanity relationship in a manner deemed to be incompatible with society's requirements, instead of keeping it moored to an established norm like normal conversation, thinking, reading, writing, humming, whistling, etc., according to accepted standards of procedure. Yet the man who talks to himself is probably no 'madder' than the one who thinks to himself; his 'madness' is simply more conspicuous on account of its audible nature, which might well indicate that the 'madman' in question is simply more extrovert or less intellectual than the habitual thinker.
However, as for those who generally do their best to 'keep in line' and remain fairly consistent with society's demands and standards, which includes the great majority of people, we shall continue to regard them as 'sane' without entirely believing it. For if they are to remain sane in the world's eyes, they must continue to cultivate their insanity as before, i.e. by taking things more or less for granted and keeping uncritical track of social requirement, as effecting and pertaining to both themselves and society in general.
As a sort of afterthought to the above, it ought to be clearly understood that insanity (as represented here by an unusual arrangement of the normal duality) and a mental breakdown are two entirely different things, since a mind which literally ceases to function - as in the cases of Baudelaire, Maupassant, and Nietzsche - should not be confused with a mind which continues to function, albeit in a highly personal and irregular way - as in the cases of Swift, de Nerval, and Ezra Pound.