Just as it is certain that one leaf is never totally the same as another, so it is certain that the concept "leaf" is formed by arbitrarily discarding these individual differences and by forgetting the distinguishing aspects. This awakens the idea that, in addition to the leaves, there exists in nature the "leaf": the original model according to which all the leaves were perhaps woven, sketched, measured, colored, curled, and painted--but by incompetent hands, so that no specimen has turned out to be a correct, trustworthy, and faithful likeness of the original model.
Friedrich Nietzsche
difference, identity, truth