So, what of Chesterton's quote? Well, he argues that an insane man has a reason for everything, and only a sane man has the freedom to something meaningless.
Necessity is a prison, therefore having a reason to do something is a type of confinement. There is no moderation in confinement: you are either in or out.
Necessity has boundaries which are easily defined and met, and so if we reduce life to necessity, we are not really being moderate. Insanity really begins when a man is overwhelmed by false necessities.
The walls of necessity are fears, which tell us not to go over the fence lest we suffer and die. Fear keeps us meeting out necessities, and so some fears are helpful.
But, moderation is not about necessity, since we can enjoy things in moderation that are not necessary.
There is a difference between eating to necessity and eating to enjoyment. It is the second, enjoyment, where moderation becomes an issue: when does my enjoyment become dangerous to me?
Moderation pulls us back from the dangerous cliff that enjoyment presents: we can lean out over the cliff to enjoy the view below, but too far and we fall. The insane, immoderate man jumps, but the man of necessity never approaches the cliff because he does not need the view in order to survive. Only the moderate man can go beyond mere existence and see the world.
Art teaches us that humans have a drive beyond necessity. Art is not necessary for survival in animals, yet archaeology teaches us that all humans have engaged in some kind of art. Art satisfies a deeper longing beyond necessity.
Art brings joy, and humans really do need to have joy beyond mere survival. people can survive without being happy, but it is rarely by intention.
But art, as with all other things not required for biological life, must be governed by moderation. Moderation keeps the drive for art from sacrificing one's existence.
The insane man is not governed by moderation, and he cannot be moderate. That's because moderation is anchored in reality, and the insane man is prisoner to his false fears. Insanity is to live only by necessity, and often by as many false necessities as real ones.
Moderation knows necessity, but not the other way around. That's why it is often hard to quantify moderation, since necessity is all about quantification.