Story of Psychology - Morton Hunt [40]
He imagined that the flow of animal spirits also powers digestion, the circulation of the blood, and respiration, and some psychological functions, like sensory impressions, the appetites and passions, and even memory. The latter, though seemingly a function of mind, he explained in mechanical terms. Much as holes in a linen cloth pierced by needles remain when the needles are removed, so repeated experiences make certain pores in the brain remain more open than others to the flow of the spirits.8 Descartes thus dispensed with Aquinas’s theory (derived from Aristotle) that the soul has “vegetative” and “sentient” as well as rational functions. In Descartes’ system it was purely rational; the other functions belong to the body.
Erroneous as his mechanical-hydraulic theory is in its details, it is impressively close to right in one major respect: it attributes the control of the muscles to impulses traveling from the brain through efferent nerves. Even more impressive is another of his guesses. He asked himself what initiates the flow of animal spirits to the muscles and again used the analogy of the royal automata, which were activated by water turned on when a visitor stepped on hidden pedals. In living creatures, he suggested, sensory stimuli play the same part by creating pressure on the sense organs; this pressure, transmitted by the nerves to the brain, opens particular valves, thereby causing bodily action of one kind or another. Descartes was thus the first to describe what would later be called the reflex, in which a specific external stimulus causes the organism to respond in a specific way.
But the mechanical-hydraulic theory did not explain consciousness, reasoning, or will. Those higher mental activities, Descartes believed, must be functions of the soul (or mind). Whence does this thinking soul get its information and ideas? He says that when it coexists with the body during life, it acquires some ideas via the body’s perceptions, passions, and memory, and it manufactures others—imaginary objects, dreams, and the like—out of remembered sensory impressions. But its most important ideas cannot come from such sources, for while he is aware of his own thinking and therefore knows that his soul exists, he never experiences his soul in a sensory fashion. The idea of the soul must be part of the soul itself. Similarly, such abstract concepts as “perfection,” “substance,” “quality,” “unity,” “infinity,” and the geometrical axioms seemed to him to be independent of sensory experience and so had to originate in the soul itself; they are innate.9
He reasonably added that such innate ideas do not exist full-blown at birth; rather, the soul has a tendency or propensity to develop them in response to experience. They are “primary germs of truth implanted by nature”; sensory impressions cause us to discover them within ourselves. For example, a child cannot understand the general truth “When equals are taken from equals, the remainders are equals,” unless you show him examples.10
His dualistic conception of body and soul presented one exceedingly difficult problem. When body and soul are locked together during life, they interact. The body’s experiences engender passions in the soul, and the soul’s thoughts and will direct the flow of animal spirits, producing voluntary movement—but where and how does the interaction take place? How can the incorporeal soul, possessing no solidity and occupying no space, connect with the corporeal