The Will to Power · Friedrich Wilhelm Nietzsche
520.
The continual transitions that occur, forbid our speaking of the "individual," etc.; the "number" of beings itself fluctuates. We should know nothing of time or of movement, if, in a rough way, we did not believe we saw things "standing still" behind or in front of things moving. We should also know just as little about cause and effect, and without the erroneous idea of "empty space" we should never have arrived at the concept of space at all. The principle of identity is based on the "fact of appearance" that there are some things alike. Strictly speaking, it would not be possible to "understand" and "know" an evolving world; something which is called "knowledge" exists only in so far as the "understanding" and "knowing" intellect already finds an adjusted and rough world to hand, fashioned out of a host of mere appearances, but become fixed _to_ the extent in which this kind of appearance has helped to preserve life; only to this extent is "knowledge" possible--that is to say, as a measuring of earlier and more recent errors by one another.
Text from Project Gutenberg, public domain in the United States.