"Nothing is beautiful, except man alone: all aesthetics rests upon this naïveté, which is its first truth. Let us immediately add the second: nothing is ugly except the degenerating man — and with this the realm of aesthetic judgment is circumscribed. Physiologically, everything ugly weakens and saddens man. It reminds him of decay, danger, impotence; it actually deprives him of strength. One can measure the effect of the ugly with a dynamometer. Wherever man is depressed at all, he senses the proximity of something "ugly." His feeling of power, his will to power, his courage, his pride — all fall with the ugly and rise with the beautiful. In both cases we draw an inference: the premises for it are piled up in the greatest abundance in instinct. The ugly is understood as a sign and symptom of degeneration: whatever reminds us in the least of degeneration causes in us the judgment of "ugly." Every suggestion of exhaustion, of heaviness, of age, of weariness; every kind of lack of freedom, such as cramps, such as paralysis; and above all, the smell, the color, the form of dissolution, of decomposition — even in the ultimate attenuation into a symbol — all evoke the same reaction, the value judgment, "ugly." A hatred is aroused — but whom does man hate then? There is no doubt: the decline of his type. Here he hates out of the deepest instinct of the species; in this hatred there is a shudder, caution, depth, farsightedness — it is the deepest hatred there is. It is because of this that art is deep."
F.W. Nietzsche - Twilight of the Idols (1895)