It is necessary to say something about Vision in the first place, if we are to have any grasp of the idea of form.
An act of vision is not so simple a matter as the student who asked her master if she should "paint nature as she saw nature" would seem to have thought. And his answer, "Yes, madam, provided you don't see nature as you paint nature," expressed the first difficulty the student of painting has to face: the difficulty of learning to see.