Vision is our dominant sense. We derive most of our information about the world-about where things are, how they move, and what they are-from the light that enters the eyes and the processing in the brain that follows. These functions are performed by all sighted animals, including ourselves, and yet we still do not understand how. Vision is also the sense about which we know the most, because of the vast amount of empirical research that has been undertaken over the years. This large body of knowledge is celebrated in most of the textbooks that have been written on visual perception; indeed, it can act as a shroud that obscures the purpose of vision from many of those who study it. We feel that textbooks tend to focus too closely on the plethora of phenomena of vision rather than on its function: They frequently reduce vision to a series of headings such as brightness, colour, shape, movement, depth, illusions, and so forth, while remaining blind to the uses to which it is put. We have tried to redress the balance a little in this book The principal focus is the function that vision serves for an active observer in a three-dimensional environment-we must be able to see where objects are if we are going to behave with respect to them. Thus the perception of location, motion, and object recognition provides the core to the book, and our intention is to make the ideas involved in their study accessible to the reader with no background in psychology. With this in mind we decided deliberately to avoid citing references in the text. This strategy might prove trying for the instructor, but it is hoped that it has the effect of making the book more readable. If it is necessary to qualify every minor point regarding the experimental base of a scientific discipline then it can not be very securely founded. Another feature we have tried to stress is the historical context in which our present studies are conducted. The history of the study of vision is as long as that of science itself, and many forces have fashioned the conceptual framework in which it operates today-our ideas have been shaped by art, optics, biology, and philosophy as well as by psychology. We need to appreciate these influences if we are to learn from, and avoid repeating, errors from the past.
The other framework that has structured this book is that of three-dimensional space: All our behaviour takes place with respect to it, and since behaviour is guided by perception it is logical that they share the same