Image on retina does not constrain the possibilities; not enough information. Human faces probably single most important visual stimulus. Whether or not we can correctly recognize important differences. Must recognize things to engage in right behaviour. Somehow know objects do not change with different orientations. Somehow able to generate name of objects. Do not need to see whole head to recognize/distinguish faces. Image on retina can be caused by an infinite number of objects. Common, but typically do not hinder environment. Viewpoint invariance: ability to recognize object regardless of viewpoint. Some of how we perceive things is in our culture, and some is in our evolution. There is enough information in the stimulus to explain our perceptions. Percept of object is sum of combining elements. Cannot explain apparent movement or illusory contours since observer sees things not present in stimulus.