Cartesian dualism emerges from Descartes's approach of radical skepticism. Wanting to know what can be determined to be absolutely true, Descartes begins by doubting all sensory perception as fundamentally external and liable to interference. Just as we understand that hallucination exists as a real phenomenon -- whereby we might "see" an object that is not really there -- we may come to understand that all the evidence obtained from eyesight may not necessarily be a valid representation of the external world. Indeed, we do not even have to refer to the pathological category of hallucination to understand what it would mean to find sensory evidence to be deceptive. In his recent book on hallucinations, the noted neuroscientist Dr. Oliver Sacks (2012) makes reference to "dreams, which one can argue are hallucinations of a sort" (xiii). Anyone who has had a vivid dream knows that they contain visual, auditory, and other sensory content which can seem very real, but which is entirely illusory. This is not a pathological phenomenon but exceptionally common for all people, and considering the way in which the existence of dreams might problematize the very concept of sensory evidence, we can understand the reasons for Descartes's radically skeptical approach. His ultimate decision, however, is that even if one may doubt the existence of a self, there is still something there which is doing the doubting: thus, Descartes establishes existence as being based on the actual act of thought. However, this does not establish the reality of the external world, so Descartes is left with an unbridgeable gulf between the mind itself (which processes perceptions et cetera) and the outside world. This gives rise to the condition of dualism, the notion that the mind represents some other form of stuff than everything else. However, as is clear in Churchland's account, the most plausible objection to Cartesian dualism is provided by materialism.
The merits of Cartesian dualism are fairly easily stated. The dualist view is the most rigorously skeptical approach to all human knowledge, and it even manages to hold up to the most outlandish possibilities -- this includes the notion that all existence might possibly be a dream (a thing which cannot be disproven, and thus cannot be ruled out), a possibility that Descartes (1999) defines in terms of "an evil genius, supremely powerful and clever, who has directed his entire effort at deceiving me" (62). This notion is perhaps most familiar to twenty-first century readers in terms of the scenario put forth by the Matrix movies, in which the notion that a human brain could be hooked up to computer-generated sensory-input feeds, which are ultimately completely deceptive on every level. The simple fact is that a radically skeptical approach has to take into account all of the ways in which the mind could potentially be deceived by apparent evidence, and this alarming possibility whereby all sensory evidence is deceptive is one that cannot be ruled out logically. Therefore, Descartes must take it into account in order to be able to base his account of the workings of the mind on the most solid possible foundation, and as a result, he retreats fully into external thought as the only proof of existence. Because this matches up to ordinary people's experience -- if we are capable of reading and understanding Descartes, we are certainly capable of having an inner life of this sort -- the idea of dualism manages to seem quite natural. The fact that it also matches up to traditional religious accounts of the idea of a soul that can survive the physical body that contains it also helps to make Cartesian dualism seem plausible: even if these religious accounts are not precisely evidence by the most rigorous standards that Descartes would apply, they help to naturalize the idea of dualism for Descartes's readers, and make it seem plausible and realistic.
However, the objections to dualism are, in Churchland's account, also persuasive, and perhaps the most persuasive alternative is provided by simple materialism. The materialist view holds that all these mental phenomena are readily explained as consequences of the physical constitution of the mind, namely the actual physical brain and its ordinary operation. The first argument in favor of the materialist view is, as Churchland (1988) emphasizes, "simplicity" (16). The notion that all matter consists of the same substance, and that all of the interior workings of mental processes might ultimately be explained through greater scientific investigation of the physical workings of the brain, is ultimately a simpler explanation of the way things work than assuming that there are two substances of different (but interacting) natures that comprise matter and mind. Expanding upon this objection in terms of the obvious scientific basis that it would entail, Churchland (1988) additionally emphasizes the "explanatory impotence" of the Cartesian dualist point-of-view (16). The materialist conception has in its favor a scientific basis, which will ultimately follow the methods of science in attempting to predict the workings of the mind. There is no such body of scientific knowledge to support the dualist point-of-view, however. As Churchland (1988) notes, "compared to the rich resources and explanatory successes of current materialism, dualism is less a theory of mind than it is an empty space waiting for a genuine theory of mind to be put in it" (19). If we follow the Cartesian position, then it becomes clear that the Cartesian separation of mind as being an independent and altogether different stuff from material reality does not permit any actual scientific investigation of what sort of stuff actually does comprise mind. We actually know less about the way the mind works if we follow Descartes -- instead it becomes a thing which is apparently set apart from rigorous scientific investigation.
This emphasis on science can go further in raising objections to the system of Cartesian dualism. For example, as Churchland (1988) additionally emphasizes, "if there really is a distinct entity in which reasoning, emotion, and consciousness take place, and if that entity is dependent on the brain for nothing more than sensory experiences as input and volitional executions as output, then one would expect reason, emotion, and consciousness to relatively invulnerable to direct control or pathology by manipulation or damage to the brain" (Churchland 20). However, anyone who has ever gotten drunk, or taken Adderall, can tell that these possible conclusions are all invalid: there are plenty of ways whereby the workings of the mind can be affected by chemicals, as well as by diseases. The gradual loss of mental function in Alzheimer's disease, for example, seems to undercut the Cartesian position, as it would seem to indicate that the functions of mind are wholly dependent upon the biological condition of the brain itself. The way in which an aging brain -- or one whose aging process is rapidly advanced by pathologies like Alzheimer's or Parkinson's -- displays gradual loss of cognitive function would not seem to support the Cartesian position that cognitive function exists as part of an independent process that somehow inhabits the material body; instead these diseases indicate that there is more evidence in favor of the materialist position that thought processes are somehow generated organically by the brain itself.
A final objection is also provided by science, although not as directly as the scientific observation of brain functions provided by neurology. This is the larger historical scientific objection which Churchland (1988) summarizes as the "argument from evolutionary history" (20). In this view, evolution is a purely physical process: over long periods of time, gradual introductions of genetic variation and mutation are affected by the process of natural selection, and ultimately result in new species. Even phenomena of extraordinary complexity, like the human eye, have evolved from much simpler forms, and there is no need in any of this evolutionary…