Hitherto we have had to do with fixed, irreversible laws, which are to be implicitly followed in order to render singing as perfect as possible. We have seen how the decline of the art of singing had to follow as a necessary consequence the non-observance of these laws. In speaking thus far of the agreeable and the disagreeable, of the beautiful and its opposite, we have had no reference to artistic feeling. We have been concerned only with direct sensuous pleasure or pain, not with æsthetic beauty. We have been occupied thus far with the _technique_ of our art--the form. But with the animating spirit of this form, the _æsthetic_, we enter upon a broader field, which, dependent upon purely psychological reasons (_Motiven_), may undergo a change, either from the general progress of mankind or from the culture of the practised artist. Thus, although from Aristotle down to Lessing and our own times the principles of beauty in all the arts are the same, yet every period, in which art has nourished, has produced works, various indeed, and corresponding to the spirit of the age, works, which, however, notwithstanding all differences, have still conformed to the demands of the principles of beauty. Thus, in architecture, sculpture, painting, music, &c., there are different styles of art, every one of which, however, has its justification and its peculiar beauty. We are not, therefore, to judge these different styles of art by the taste and ideas (_Auffassung_) of the present, but by the character of the times that produced them. Although the mode of thinking may vary in accordance with the different stages of culture of individuals and mankind, there are, nevertheless, certain principles of beauty which all nature announces.