For him, extension in space is the only objective quality of matter, and space is in turn reducible to number through the coordinate system. He grasped that the brain is the organ of perception, and that the afferent nerves relay a display of signals, not a copy of the world. Descartes had studied the optics of vision by dissecting an eye in such a way as to actually observe, from behind, the image projected upon the retina. For in no way does the mere transmission of an optical image in the eye explain the process of vision. If it did, we might as well imagine that television sets can see.
Vision is rather an intentional process of interpreting the image.
To the brain, an image is a pattern of signs to be given meaning rather than a copy of the world. This points to another way in which the camera metaphor is misleading. In looking at an object and at a picture or image of that object, we can see that they are isomorphic, and that there is an analogy between the lens of the camera and that of the eye. But this tells us nothing of intentional mappings, represented by neural transformations in the brain, as opposed to optical transformations taking place in the eye. We cannot conclude, on the grounds of the camera metaphor, that there is an isomorphism between our experience of the object and the object itself.