Theory of Sense Perception


As the new “mechanical philosophy” of Descartes and others replaced the Aristotelian physics, the theory of sensory qualities had to undergo substantial change. This was especially true for what came to be known as the secondary qualities (in the terminology of Robert Boyle and John Locke). The secondary qualities include colors, sounds, odors, tastes, and tactile qualities such as hot and cold. The Aristotelians maintained that these qualities exist in objects as “real qualities” that are like instances or samples of the quality as experienced. A red thing possesses the quality red in just the same way it possesses a shape: it simply is red, and we experience that very redness when we see a red object.

Descartes sought to replace “real qualities” with a mechanistic account of qualities in objects. He rendered light as a property of particles and their motions: it is a “tendency to move” as found in a continuous medium and radiating out from a luminous body. When light strikes an object, the particles that constitute light alter their rotation about their axis. “Spin” is what makes light have one color rather than another. When particles with one or another degree of spin interact with the nerves of the retina, they cause those nerves to jiggle in a certain way. This jiggling is conveyed to the brain where it affects the animal spirits, which in turn affect the mind, causing the mind to experience one or another color, depending on the degree of spin and how it affects the brain. Color in objects is thus that property of their surface that causes light particles to spin in one way or another, and hence to cause one sensation or another. There is nothing else in the surface of an object, as regards color, than a certain surface-shape that induces various spins in particles of light.


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