The major problem in visual perception is that what people see is not simply a translation of an image on the retina. For one thing, we see the world right side up, even though the image on the retina is upside down (because it has passed through the lens). Therefore, it is difficult to explain what happens to create what we actually see. The key, which took centuries to be appreciated, is that the brain works on the data from the eyes, and marries it with memories and guesses, all at lightning speed. The result is an experience of the world which looks to each person as if it were simple reality. However, although based on reality, it is actually a mental construct, built by the brain.