cal basis but then follow exceedingly intricate pathways, bringing in new forms of information at each step, and arriving finally in the realm of subjective experience. Thus the workings of the senses pass at some point beyond the reach of the experimental scientist, because the results can never be reproduced exactly; once information from the brain's association cortex is brought in, the body is no longer simply taking part in some reaction predictable from the laws of physics. Rather, the mind is perceiving something, and the perception is uniquely shaped by that perceiving mind, at that moment.

Nevertheless, scientific research can do a great deal to unravel the pathways of perception, clarifying how and along what lines the information is organized. Vision, the best understood of the perceptive systems, can be explained with confidence well beyond the action of light on the rods and cones of the retina. And some of the mechanisms of this system turn out to be active in other aspects of our conscious behavior as well—namely, in spatial perception and the specific state of mind known as attention.

FIGURE 7.1. Explaining the workings of our five senses is a challenge that has been taken up from time to time by natural historians or philosophers. For example, in the seventeenth century, Réné Descartes hypothesized that we are able to see because the nerves project an object from the eyes into the brain, where it is perceived by the soul. Source: Réné Descartes, 1664. Un Traitté de la Formation du Foetus du Mesme Autheur. The National Library of Medicine.