Louis Sullivan: Use

Source: Kindergarten Chats (1918), Ch. 36 : Another City
Context: We are rounding out our absorbing study of Democracy. Thus, turning slowly upon the momentous axis of our theme, are we coming more and more fully into the light of our sun: the refulgent and resplendent and life-giving sun of our art — an art of aspirant democracy! Let us then be on our way; for our sun is climbing ever higher. Let us be adoing; lest it set before we know the glory and the import of its light, and we sink again into the twilight and the gloom from which we have come.

Emotional Architecture as Compared to Intellectual (1894)
Context: Man, by means of his physical power, his mechanical resources, his mental ingenuity, may set things side by side. A composition, literally so called, will result, but not a great art work, not at all an art work in fact, but merely a more or less refined exhibition of brute force exercised upon helpful materials. It may be as a noise in lessening degrees of offensiveness, it can never become a musical tone. Though it shall have ceased to be vulgar in becoming sophistical, it will remain to the end what it was in the beginning: impotent to inspire — dead, absolutely dead.
It cannot for a moment be doubted that an art work to be alive, to awaken us to its life, to inspire us sooner or later with its purpose, must indeed be animate with a soul, must have been breathed upon by the spirit and must breathe in turn that spirit. It must stand for the actual, vital first-hand experiences of the one who made it, and must represent his deep-down impression not only of physical nature but more especially and necessarily his understanding of the out-working of that Great Spirit which makes nature so intelligible to us that it ceases to be a phantasm and becomes a sweet, a superb, a convincing Reality.

The Tall Office Building Artistically Considered (1896)
Context: 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.
Unfailingly in nature these shapes express the inner life, the native quality, of the animal, tree, bird, fish, that they present to us; they are so characteristic, so recognizable, that we say simply, it is "natural" it should be so. Yet the moment we peer beneath this surface of things, the moment we look through the tranquil reflection of ourselves and the clouds above us, down into the clear, fluent, unfathomable depth of nature, how startling is the silence of it, how amazing the flow of life, how absorbing the mystery! Unceasingly the essence of things is taking shape in the matter of things, and this unspeakable process we call birth and growth. Awhile the spirit and the matter fade away together, and it is this that we call decadence, death. These two happenings seem jointed and interdependent, blended into one like a bubble and its iridescence, and they seem borne along upon a slowly moving air. This air is wonderful past all understanding.
Yet to the steadfast eye of one standing upon the shore of things, looking chiefly and most lovingly upon that side on which the sun shines and that we feel joyously to be life, the heart is ever gladdened by the beauty, the exquisite spontaneity, with which life seeks and takes on its forms in an accord perfectly responsive to its needs. It seems ever as though the life and the form were absolutely one and inseparable, so adequate is the sense of fulfillment.

Emotional Architecture as Compared to Intellectual (1894)

Education (1902)