Interesting as Cartesian coordinates are, that is not our concern with Descartes. Rather, we are much more interested in Descartes the philosopher and his solution to the mind-body problem. That philosophical problem wrestles with the question of reality itself. We could all probably agree about external aspects of reality. For example, the room you are in is real, the manual is real, the page is real. We can touch them, see them, bite them, and so on. When we close the book, we all agree that we can no longer see the page. So, external reality, or the body part of the mind-body problem, is not really the issue. The issue really confronts us when we consider our internal view of external reality, in other words, how our mind represents external reality and whether it creates de novo other aspects of internal reality. For example, ask yourself what color a tennis ball is. I will make it a multiple choice question: Is it yellow, is it green, or is it a yellowish green? Now go ask your friends and acquaintances the same question. Guess what? You will find three answers, because different people see the same physical object in different ways. (I see them as green, by the way. For years, I could not understand why the cans were labelled "Yellow Tennis Balls". My wife sees them as yellow, and interestingly, my first son sees them as green.) There is a physiological explanation; it has to do with genetic differences in eye pigments. We will discuss color vision in Chapter 4.
Back to the mind-body problem, the fact that different people perceive tennis balls differently shows us that internal realities vary. But the mind-body problem is deeper than that when one begins to consider the interactions between mind and body. Before Descartes, the problem was fuzzy, convoluted. People believed that mental powers routinely affected the physical world. For example, the witch down the road could kill your chickens by hexing them, so you had to be nice to her. Today, we do not think that way, or do we? Consider the following: We carry brides across thresholds, we put pine trees on top of buildings when we finish them, and we throw salt over our shoulders. But, most of us only pay lip service to such behaviors. We do not really believe that our marriage will last if we carry our bride across the threshold, or that the building will fall if we do not top it, or that bad luck will follow if we fail to throw the salt.
In the Middle Ages, however, people did believe strongly in those kinds of interactions between internal and external reality. Further, they believed that both types of realities were completely and totally bound together. Descartes destroyed that type of thinking by proposing that mind and body were separable. He said that they existed separately but that they did interact from time to time. For that reason, we call Descartes' solution to the mind-body problem interactionism.
In many ways, one can argue that interactionism led directly to the creation of the modern world. By separating mind and body, Descartes made it possible to study the two separately, and that is exactly what happened. The route made possible by studying the body or external reality, is called mechanism. Today, we pay more attention to the engineer's analysis of a building than we pay to whether or not a building was topped by a pine tree during its construction. If the building were to collapse, we would look for the engineer, not for the person who rigged the pine tree. In short, science and technology have flourished by ignoring, at some level, the mind half of the mind-body problem.
The route to internal reality or to the mind has been much more difficult, and in many ways has not been attempted. The study of internal reality is called phenomenology. Psychology has always been interested in the mind. Note that physics and chemistry have no such interest. Chemists need not consider the mental state of electrons when they formulate their equations, for example. Psychologists, however, do have to consider mental states to some extent, and that requirement makes psychology more complex, by definition, than either physics or chemistry. That complexity also helps explain the relative state of development of psychology as a field. Being more complex, psychology has lagged behind the "hard sciences ." So, today psychologists are interested in both internal and external reality, and they owe that interest to Descartes and his interactionist solution to the mind-body problem.