In the previous two sections we discussed walking and turning on a stage to illustrate how physical action can be analyzed in terms of the movement of bones and joints. In this section we shall analyze vision. In the first chapter we discussed how action is required to make purposive vision and perception possible. In the second chapter we examine visual input through the eyes.
We are restricting our analysis to single-focus eyes. This restriction still allows us to include all vertebrates.
The interaction between the eye and the physical world is through optics. The eye contains a lens that focuses incoming light onto a light-sensitive surface, the retina, that converts the optical signal into an electrical signal.
The closest analogy is the image of a video camera.