99. That is her work then--the giving of Form; then the separately Apolline work is the giving of Light; or, more strictly, Sight: giving that faculty to the retina to which we owe not merely the idea of light, but the existence of it; for light is to be defined only as the sensation produced in the eye of an animal, under given conditions; those same conditions being, to a stone, only warmth or chemical influence, but not light. And that power of seeing, and the other various personalities and authorities of the animal body, in pleasure and pain, have never, hitherto, been, I do not say, explained, but in anywise touched or approached by scientific discovery. Some of the conditions of mere external animal form and of muscular vitality have been shown; but for the most part that is true, even of external form, which I wrote six years ago. "You may always stand by Form against Force. To a painter, the essential character of anything is the form of it, and the philosophers cannot touch that. They come and tell you, for instance, that there is as much heat, or motion, or calorific energy (or whatever else they like to call it), in a tea-kettle, as in a gier-eagle. Very good: that is so, and it is very interesting. It requires just as much heat as will boil the kettle, to take the gier-eagle up to his nest, and as much more to bring him down again on a hare or a partridge. But we painters, acknowledging the equality and similarity of the kettle and the bird in all scientific respects, attach, for our part, our principal interest to the difference in their forms. For us, the primarily cognizable facts, in the two things, are, that the kettle has a spout, and the eagle a beak; the one a lid on its back, the other a pair of wings; not to speak of the distinction also of volition, which the philosophers may properly call merely a form or mode of force--but then, to an artist, the form or mode is the gist of the business."[23]