The Will to Power · Friedrich Wilhelm Nietzsche
565.
Qualities are our insurmountable barriers; we cannot possibly help feeling mere _differences of quantity_ as something fundamentally different from quantity--that is to say, as _qualities,_ which we can no longer reduce to terms of quantity. But everything in regard to which the word "knowledge" has any sense at all, belongs to the realm of reckoning, weighing, and measuring, to quantity whereas, conversely, all our valuations (that is to say, our sensations) belong precisely to the realm of qualities, _i.e._ to those truths which belong to us alone and to our point of view, and which absolutely cannot be "known." It is obvious that every one of us, different creatures, must feel different qualities, and must therefore live in a different world from the rest. Qualities are an idiosyncrasy proper to human nature; the demand that these our human interpretations and values, should be general and perhaps real values, belongs to the hereditary madnesses of human pride. The "real world," in whatever form it has been conceived hitherto--was always the world of appearance _over again._
Text from Project Gutenberg, public domain in the United States.