It has been observed, that in order to enjoy natural objects aesthetically, we should withdraw them from their external and historical reality, and separate their simple appearance or origin from existence; that nature is beautiful only for him who contemplates her with the eye of the artist; that zoologists and botanists do not recognize beautiful animals and flowers; that natural beauty is discovered; that, without the aid of the imagination, no part of nature is beautiful, and that with such aid the same natural object or fact is now expressive, according to the disposition of the soul, now insignificant, now expressive of one definite thing, now of another, sad or glad, sublime or ridiculous, sweet or laughable; finally, that natural beauty, which an artist would not to some extent correct, does not exist.

What is the underlying theme of the passage?