The medium for transmitting light or allowing observer movement. The receptor which transduces energy. The nerves which transmit impulses. The brain with reception, abstraction memory and movement areas. The action system which can change the relations between parts of the chain.
Senses find how surfaces vary, and when they vary, and thereby find where they vary.
Surfaces can be flat or curved. They can face to or away from a vantage point. Virtually all objects whose shapes can be perceived are combinations of surfaces. Surfaces form corners - concave or convex. Convex corners can form occluding edges.