7. Consider the act of ocular vision:
There are two elements here; there is the form perceptible to the sense and there is the medium by which the eye sees that form. This medium is itself perceptible to the eye, distinct from the form to be seen, but the cause of the seeing; it is perceived at the one stroke in that form and on it and, hence, is not distinguished from it, the eye being held entirely by the illuminated object.