"Light reflects from the surfaces of objects in the physical environment and passes into the eyes, where it irritates the cells of the retinas in such a way that impulses are triggered in the optic nerves. The nerves convey the impulses to the part of the cerebral cortex which processes visual data, where they stimulate certain sorts of brain activity. As a result, in ways yet mysterious to science and philosophy, coloured motion pictures arise in the consciousness of the subject of these events, representing the world outside his head."...
Only a little reflection shows that some of the properties we seem to perceive in objects are in fact not 'in the objects themselves' but are the artefact of the perceptual relation. The colour, taste, smell, sound, and texture of objects vary according to the condition of the perceiver or the conditions under which they are perceived. The standard examples are legion. Grass is green in daylight, grey or black at night; tepid water feels warm to a cold hand, cool to a hot hand; objects look large from close by, small from far away... These perceptual relativities are invoked by the sceptic to raise questions not just about whether pereception is a reliable source of information about the world, but, further, whether the world can properly be said to exist independently of perception at all.
Anthony C. Grayling, Scepticism.
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