In 150 years the science of perception has taught us that the way we appreciate the world around us is as much dependent on our expectations, our experiences, our inferences, as it is on the hard evidence of images on our retinas or vibrations in our ears. Remember the occasions when you have seen a face in the flickering flames of a fire, or been certain that you saw a person in the distance as you walked along at night – only to discover that the face in the fire disappears with the next burst of flame and the person in the dark is just a letterbox.