The original text comes from "The Tall Office Building Artistically Considered" by Louis Sullivan. I've edited down the text considerably to reveal what I feel are the best parts.
All of these critics and theorists agree, however, positively, unequivocally, in this, that the tall office building should not, must not, be made a field for the display of architectural knowledge in the encyclopaedic sense; that too much learning in this instance is fully as dangerous, as obnoxious, as too little learning; that miscellany is abhorrent to their sense …
To this latter folly I would not refer were it not for the fact that nine out of every ten tall office buildings are designed in precisely this way, in effect not by the ignorant, but by the educated. It would seem indeed as though the "trained" architect, when facing this problem, were beset at every story, or at most, every third or fourth story, by the hysterical dread lest he be in "bad form". Lest he be not bedecking his building with sufficiency of quotation from this, that or the other “correct” building in some other land and some other time. Lest he be not copious enough in the display of his wares. Lest he betray, in short, a lack of resource. To loosen up the touch of this cramped and fidgety hand, to allow the nerves to calm, the brain to cool, to reflect equably, to reason naturally, seems beyond him; he lives, as it were, in a waking nightmare filled with the disjecta membra of architecture. The spectacle is not inspiriting. …
All things in nature have a shape, that is to say, a form, an outward semblance, that tells us what they are, that distinguishes them from ourselves and from each other. …
It is the pervading law of all things organic and inorganic, of all things physical and metaphysical, of all things human and all things superhuman—of all true manifestations of the head, of the heart, of the soul—that the life is recognizable in its expression, that form ever follows function. This is the law.
Shall we, then, violate daily this law in our art? Are we so decadent, so imbecile, so utterly weak of eyesight, that we cannot perceive this truth so simple, so very simple? …
… then, it may be proclaimed, that we are on the high road to a natural and satisfying art, an architecture that will soon become a fine art in the true, the best sense of the word, an art that will live because it will be of the people, for the people, and by the people.
—Louis H. Sullivan.