In his First Meditation, Descartes begins with his method of universal doubt, calling into question everything which can be called into question. He begins by noting that sometimes the conclusions he has drawn from sense-experience have been erroneous. But this justification for doubt leaves quite a few propositions untouched, for, Descartes tells himself,

"although the senses may sometimes deceive us about some minute or remote objects, yet there are many other facts as to which doubt is plainly impossible although these are gathered from the same source: e.g. that I am here sitting by the fire, wearing a winter cloak, holding this paper in my hands, and so on." (62)

There is, however, an even stronger justification for doubt to be had. Descartes considers the possibility of a deceitful omnipotent being, who could apparently deceive Descartes even about the simplest of judgments. Such a being could, Descartes thinks, make him believe that there are corporeal objects, that there is time and space in which these objects exist, etc., even though there are no such things. Such a being could even deceive Descartes, so he thinks, about simple judgments such as that two and three make five, or that a square has four sides. And since Descartes has yet no reason to completely rule out this possibility, at the end of the First Meditation, Descartes seems to be in doubt about everything.

Over the course of Meditations II-V, Descartes believes that he has established, from the raw phenomena and a few principles which evidently weren't so dubitable after all, that he exists, that he is essentially a conscious substance, that God exists and is not a deceiver, and that any judgment which is "clear and distinct" is indubitable. It remains for him in Meditation VI to examine those judgments which he initially called into question at the beginning of the Meditation I: the existence of corporeal objects and the validity of judgments based on sense-perception.

Among the judgments about corporeal objects which Descartes is concerned with in the Sixth Meditation is the judgment that among the corporeal bodies which exist is one which is, "by a special title" (112), his body, one which is intimately connected in some way with the conscious substance which he essentially is. He notes first that, previous to the doubt justified in the First Meditation, he had some empirical reason to call one of these bodies his. For there seemed to be one of these bodies which he was unable to separate himself from, as he could separate himself from other bodies. Furthermore, it was this body which at least some of his sensations, such as pains, seemed to be located in. And these sensations which appear to be in this body seem to be causally connected with certain judgments in his understanding. A twitching in the stomach is invariably followed by the desire that he eat, a dryness in the throat by a desire that he drink. A sadness in the understanding invariably follows upon a pain in this body, and happiness upon a pleasant sensation in this body (113).

Now the judgment that he has a body is among those called into question by his strongest method of justification for doubt found in the First Meditation, the deceitful God hypothesis, but by this time Descartes has come to what he holds to be a certain conclusion that will enable him to escape doubt about this judgment. In the Third Meditation, Descartes comes to the certain conclusion that God exists and is not a deceiver. This does not mean that he can never be incorrect in his judgments, but three important conclusions do follow from this. First, any clear and distinct judgment he makes, such as the abstract reasonings of pure mathematics, are certain. Second, he is capable of correcting any incorrect judgments that he makes. And third, he cannot, in general, be deceived about what is beneficial and harmful to himself, for "sense-perceptions were given me by nature [God] properly for the sole purpose of indicating to the mind what is good and bad for the whole of which the mind is a part" (115).

Among the clear and distinct judgments he makes are judgments about corporeal bodies, since it is the properties of these, extension, shape, motion, etc., that are the object of purely mathematical reasoning. It is not a clear and distinct judgment that corporeal objects exist, but it is clear and distinct that if such objects exist, there are certain relationships among their properties. Furthermore, Descartes notes, the sensations which are the origin of these judgments are not under the control of his will. As Descartes puts it, his power of sensation is a passive power. Descartes then relies on a principle which he has not yet used in the meditations: that if a power is passive, i.e. not under the control of his will, the active source of the objects toward which that power is directed, in this case the objects of sensation, must be outside himself (115). It should be noted that in using this principle Descartes directly contradicts something he had said in the Third Meditation: "Again, those these ideas [of corporeal objects] do not depend on my will, it does not necessarily follow that they proceed from external objects" (79). Nevertheless, by use of this principle, Descartes determines that his ideas of external objects (or at least the sensations from which these ideas arise) originate from some source external to himself.

There then seem to be two possibilities for the origin of these ideas of corporeal objects: either they originate from actual corporeal objects, or they originate from some being, possibly God, that contains the reality of corporeal objects in some higher form. This follows from his principle, first made explicit in the Third Meditation, that the source of an idea must be a cause containing at least as much inherent reality as the idea contains representative reality (82). Descartes eliminates the second disjunct of this proposition by observing that God is not a deceiver, therefore he would not himself deceive Descartes so, nor would He allow some other higher being to so deceive him. Descartes has thus proved, by disjunctive argument (modus tollendo ponens) that corporeal bodies exist. And since he cannot be mistaken about his clear and distinct judgments, these corporeal bodies have the properties of extension, shape, motion, and duration, just as he judges them to have.

Given that corporeal bodies exist, Descartes notes that among these bodies there is one which he judges to be closely linked with his mind, and that he judges certain of these bodies to be beneficial to him and others to be harmful to him. These judgments fall under the class of judgments which God, since He is not a deceiver, would not deceive him about. So despite his initial skepticism about corporeal objects in the First Meditation, Descartes holds it certain that he has a body, and that, at least in general, he is correct about what other corporeal bodies are beneficial and harmful to him.

Descartes notes, however, that sometimes persons are deceived even about such important matters as whether certain corporeal objects are beneficial or harmful. He gives the example of a person with dropsy, which causes a dryness of the throat, bringing about a desire to drink. Drinking liquids, however, aggravates the disease; the person with dropsy has been deceived about what is harmful, something which it seems a non-deceitful deity should not permit. Descartes must, in light of this evidence, defend his thesis that God is not deceitful, or else the possibility of knowledge will be threatened once again by the deceitful-God hypothesis. To defend God against the charge of being deceitful in such important matters, Descartes uses his observation that he has a body which is his own: his mind is not affected by all matter, but only by the brain, "and perhaps only by one small part of that" (121).

Descartes observes that corporeal objects are such that any motion, call it D, that could be effected by a causal chain A ­> B ­> C ­> D, could also be affected by a causal chain B ­> C ­> D (where B is the effect of some cause other than A), without A ever having happened. Descartes gives the example of a rope, and notes that if one end may be moved by pulling on the other end, it may just as well moved by pulling in the middle. As long as the understanding is connected at only one point to the realm of corporeal objects, there remains the possibility of such causal chains giving rise to incorrect judgments. Given that we are such a mixture of mind and matter, God cannot prevent error happening from time to time. Consider again the case of the man with dropsy. Normally, one's desire to drink is caused by the following sequence of events: need for water ­> physical dryness in throat ­> sensation of dryness in throat ­> desire to drink. In the case of the man with dropsy, the causal sequence runs thus: dropsy ­> physical dryness in throat ­> sensation of dryness in throat ­> desire to drink. Physical dryness in the throat can be the effect of at least two different causes, the need for water, and dropsy. Since there is nothing God can do about error arising from such sources, God is not to blame for this error, and is not deceitful. Descartes notes, however, that God, because of his non-deceitful nature, has set up the world so that we are correct most of the time about what is beneficial and what is harmful.

Now it may seem strange, or even contradictory, to say that the mind is "united to, and as it were mixed up with, the body" (117), and at the same time claim that there is this "great difference between mind and body" (121). Strange I will grant, but when we examine Descartes account of the differences between mind and body, there does not seem to be any actual contradiction present. The first thing Descartes (in the Second Meditation) says about mind and body is that the mind is better known than body. This in itself does not give rise to a contradiction. It would probably be correct to say that prior to twentieth century physics, matter was better understood than light, but this in no way precludes there being a certain intimate connection between light and matter, viz. that there can be causal interactions between the two. The relationship of 'being known better than' is an epistemic relationship, which should not, it seems, preclude the possibility of causal relationships. And, as far as I can tell, it is the causal relationships between mind and body which for Descartes constitute this intimate relationship between mind and body. The second claim Descartes makes regarding this great difference between mind and body is that whereas corporeal objects are essentially extended, mental substance is essentially not. This may seem somewhat problematic for Descartes; for, one might ask, how can a non-extended substance causally interact with an extended substance? However, if this is a problem for Descartes, it is also a problem for contemporary physics. For as Paul Churchland points out, "our best current theories describe the electron as a point-particle with no extension whatever" (Matter and Consciousness, p. 9). And surely no one would want to deny that electrons can interact causally with extended objects. Descartes third important claim, which seems to follow from his second claim, is that corporeal objects are always divisible, whereas mind is indivisible (121). But again, this does not raise in any way a contradiction. A Democritean atomist, who holds that "atoms" are indivisible, or a modern physicist who holds that quarks and leptons are the ultimate indivisible constituents of reality, need not hold that atoms or quarks or leptons cannot causally interact with divisible substances.

It is true that Descartes, or any other substance dualist for that matter, would owe us an account of the nature of the causal interaction between mind and matter in order to present a complete theory of mind. But Descartes has no pretensions, at least in the Meditations, of presenting such a theory, and his claim both that mind and matter are "greatly different" and that they are causally connected in no way leads him into a contradiction.

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