René Descartes

René Descartes (1596-1650) was a seventeenth century French philosopher whose conception of the nature of human beings and of animals had a strong influence on later thinking about learning and behavior. His major ideas in this regard include what is known today as the reflex arc, the brain as the seat of the soul and mental processes, the idea that many mental and behavioral phenomena can be understood in terms of purely physical causes, and the notion that we come into the world with certain preexisting knowledge.

Descartes and the Reflex Arc

Descartes' system of natural philosophy attempted to account for all natural phenomena in terms of matter in motion. In this conception, the actions of our bodies and those of animals can be understood purely in terms of physical cause and effect (Aristotle's efficient causation). To account for animal behavior and for the involuntary, reflexive behavior of human beings, Descartes suggested that stimuli impinging on various sense organs were capable of setting up vibrations in those organs. Those vibrations in turn tugged on tiny fibers which Descartes imagined ran through the nerves connecting the sensory organs to the brain. In the brain, the tugging of the fibers opened small valves, which then allowed the "animal spirits" (cerebral-spinal fluid) within the brain and spinal cord to drain down the hollow tubes of nerves running to the muscles. As the fluid entered the muscles, the muscles fattened up and shortened, thus producing movement in the limbs or other structures to which the muscles were attached. In this way, a boy whose finger accidentally came into contact with a flame would instantly react to the stimulus of the fire by contracting the appropriate muscles that pulled the finger out of the flame, thus producing the response. The entire chain of events took place automatically, without conscious intervention or will, purely as a result of a physical chain of cause and effect running from stimulus, down the nerve to the brain, then back out again along another nerve to muscle, and thus to response.

Descartes imagined that animal behavior could be entirely accounted for by such automatic, reflexive mechanisms. Each new stimulus impinging on the animal's senses would generate new, reflexive movements; these in turn would reorient the animal's sensory receptors, allowing new stimuli to fall upon them, initiating yet other movements, and so on. In this view, the animal was merely a biological automaton, without real consciousness and without a will.

In similar fashion, Descartes proposed mechanisms to account for bodily processes ranging from digestion through growth to reproduction, and in so doing completely eliminated the need for the nutrititive (vegetative) and sensitive "souls" Aristotle had invented to account for these functions. He could not, however, bring himself to eliminate Aristotle's rational soul from his conception of the human being. I describe this conception next.

Descartes' Interactive Dualism

Descartes proposed that voluntary actions (by definition only available to human beings) were initiated by the human soul, which he viewed as separate from the body and interacting with it at a specific point within the brain called the conarium (because of its somewhat pine-cone-like shape; today this structure is known as the pineal gland). Surrounded by animal spirits, the conarium was in the ideal location to receive vibrations conveyed to it through the fluid from various sensory organs. These vibrations were somehow sensed by the soul and interpreted as sensory experiences such as vision or hearing. Thus, sensory information being conveyed by the nerves to the brain was made available to the soul. When the soul decided to act, it could gain control over the movements of the body by somehow pushing the conarium ever-so-slightly in one direction or another, thus altering the flow of animal spirits within the brain and directing those spirits down the nerve-tube leading to one muscle or another. In this way the soul could initiate voluntary action. Literally, Descartes preserved Aristotle's purely rational soul in his model of the human being. The concept of the human being as having a dual nature -- body and spirit -- was not new with Descartes, but this particular way of combining them, involving as it does a two-way interaction via the conarium in which body affects soul and soul affects body, was. Today it is referred to as interactive dualism.

Although you may think that Descartes left to the soul all those functions we normally ascribe to the mind, in reality the soul in Descartes' scheme provided only some of those functions: it provided consciousness, the ability to think and reason, and, perhaps most importantly, the ability to think and act independently of the physical causes affecting the body. In this way Descartes preserved within his model a human capacity for free will, in contrast to the determinate system of cause and effect assumed to drive the nonhuman animal's behavior.

In Descartes' model, the human soul did not possess a memory, but rather relied on the physical structure of the brain to store its experiences. Vibrations of the animal spirits conveying sensory information, Descartes suggested, alter the shape of the soft inner walls of the brain much as a stylus leaves an impression in soft wax. When the soul needed to remember something, it sent out vibrations from the conarium; the echoing vibrations, shaped by the impressions sculpted into the walls of the brain by previous experience, could be interpreted by the soul in the same way as primary sensory experiences were; these return vibrations would be the stored memories of past sensory experiences. Emotions, too, were viewed as products of the brain rather than properties of the soul. When the body became agitated by strong sensory experience, the soul could interpret this agitation as an emotional experience, but the emotion itself originated in the body.

Today we know that Descartes' conceptions of the bodily mechanisms involved in the reflex arc, sensing, and voluntary action were incorrect -- they are not hydraulic systems, for example. Descartes did the best he could with the limited knowledge available to him at the time. That he got the mechanism wrong, however, does not detract from the importance of his thinking, for in Descartes' model we have for the first time a serious attempt to account for the animal's and the human's ability to move about, to sense and perceive, to remember, and to experience emotions in purely physical terms, without the intervention of some mystical non-physical life-force. This conception set the stage for an improved understanding of the body and brain as physical mechanisms, governed by physical laws.

Descartes' Nativism

Descartes believed that we do not come into this world completely devoid of any conceptions about it. Basing his opinion mainly on introspection, he suggested that the human soul at birth already comes equipped with an innate understanding of certain concepts, including the concepts of perfection, time, and (of all things) the geometrical axioms. By the latter I suspect that Descartes meant an innate comprehension of the dimensions of space and perhaps of certain optical rules such as those involving perspective.

The idea that we come into the world with certain pre-knowledge about it is called nativism. The contrasting idea -- that we come into the world completely ignorant and must learn everything as a result of sensory experience -- is termed empiricism. Near the end of the eighteenth century, Frances Galton was to characterize the continuing debate over how much of an ability is innately given and how much due to learning or experience as the debate of nature versus nurture, where "nature" stands for those abilities that are conferred on the individual by inheritance and "nurture" stands for those that develop as a result of experience. Descartes was, to at least some degree, on the "nature" side of this debate.