In the Book of Optics (1011–22 AD), Ibn al-Haytham argued that vision occurs in the brain, and that personal experience has an effect on what people see and how they see, and that vision and perception are subjective. Arguing against Ptolemy 's refraction theory for why people perceive the Sun and Moon larger at the horizon than when they are higher in the sky, he redefined the problem in terms of perceived, rather than real, enlargement. He said that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. Critically, Ibn al-Haytham said that judging the size of an object depends on its judged distance: an object that appears near appears smaller than an object having the same image size on the retina that appears far. With the overhead Moon, there is no uninterrupted sequence of intervening bodies. Hence it appears far and small. With a horizon Moon, there is an uninterrupted sequence of intervening bodies: all the objects between the observer and the horizon, so the Moon appears far and large. Through works by Roger Bacon, John Pecham, and Witelo based on Ibn al-Haytham's explanation, the Moon illusion gradually came to be accepted as a psychological phenomenon, with Ptolemy's theory being rejected in the 17th century. For over 100 years, research on the Moon illusion has been conducted by vision scientists who invariably have been psychologists specializing in human perception. After reviewing the many different explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug concluded "No single theory has emerged victorious".