It takes but a moment’s thought to realise that obtaining that knowledge is no easy matter. The brain is only interested in obtaining knowledge about those permanent, essential or characteristic properties of objects and surfaces that allows it to categorise them. But the information reaching the brain from these surfaces and objects is in continual fl ux. A face may be categorised as a sad one, thus giving the brain knowledge about a person, in spite of the continual changes in individual features or in viewing angle or indeed in the identity of the face viewed; or the destination of an object may have to be decided by its direction of motion, regardless of its speed or distance. An object may have to be categorised according to colour, as when judging the state of ripeness of an edible fruit. But the wavelength composition of the light refl ected from an object is never constant; instead it changes continually, depending upon the time of day, without entailing a substantial shift in its colour. The ability of the brain to assign a constant colour to a surface or a constant form to an object is generally referred to as colour or object constancy. But perceptual constancy is a much wider phenomenon. It applies as well, for example, to faces that are recognisable when viewed from different angles and regardless of the expression worn. There is also what I shall call situational constancy, when the brain is able to categorise an event or a situation as a festive or a sad one, and so on, regardless of the particular event. There is even a narrative constancy when, for example, the brain is able to identify a scene as the Descent from the Cross, regardless of variations in detail or the style of the painting. The brain, in each case, extracts from the continually changing information reaching it only that which is necessary for it to identify the characteristic properties of what it views; it has to extract constant features in order to be able to be able to obtain knowledge about them and to categorise them. Vision, in brief, is an active process depending as much upon the operations of the brain as upon the external, physical, environment; the brain must discount much of the information reaching it, select from that information only that which is necessary for it to be able to obtain knowledge about the visual world and compare the selected information with its stored record of all that it has seen. A modern neurobiologist should approve heartily of Matisse’s statement8 that “Voire, c’est déja une operation créatrice, qui exige un effort”.