Incapacity to appreciate certain types of beauty may be the condition sine qua non for the appreciation of another kind; the greatest capacity both for enjoyment and creation is highly specialized and exclusive, and hence the greatest ages of art have often been strangely intolerant. The invectives of one school against another, perverse as they are philosophically, are artistically often signs of health, because they indicate a vital appreciation of certain kinds of beauty, a love of them that has grown into a jealous passion.