" 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. "