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Summary
The Meditator muses that he has been puzzled as to why his mind seems particularly attached to one particular body, which he calls his own. Why does he feel pain and tickling in this body but not in any body external to it? And why should a tugging in the stomach of that body suggest to his mind that he should eat, since there is no obvious connection between the tugging and the decision to eat? He concludes that he is inclined by nature to assume the things he does about his body and about the world external to it, since he accepts these assumptions prior to developing any arguments regarding them.
Having questioned these assumptions in the First Meditation, he finds that there is plenty of reason to doubt that material things are the way he is naturally inclined to assume they are. However, he believes he is now well enough equipped that he needn't doubt their existence entirely. First, he clearly and distinctly perceives that he is, in essence, only a thinking thing. Body is essentially extended and mind is non-extended, so he can conclude that he really is distinct from his body and could exist without it.
The Meditator reasons that imagination and sensory perception are modes of thought. He could conceive of himself without imagination or sensory perception, so they are not essential to him, but imagination and sensory perception could not exist without a mind to contain them. Similarly, there are modes of extension that cannot exist without a body to contain them.
Sensory perception is a passive faculty, and, as the Meditator has asserted before, there must be some active cause that creates sensory perceptions and this cause must reside outside of him. Either it could be other bodies with as much formal reality as the sensory perceptions have objective reality or it could be God or some other being capable of creating these perceptions. The Meditator is naturally inclined to suppose that sensory perceptions are created by things which resemble those perceptions, and he would be deceived if the perceptions were caused by some other means. Since God is no deceiver, God would not have misled the Meditator into thinking there are material objects if there were not, so the Meditator concludes that material objects must exist. His perception of most properties of material objects is confused and obscure, so his perception of them might not be perfect, but he can at least be certain of those properties that he perceives clearly and distinctly.
The Meditator next considers those ideas about body that he perceives only confusedly and obscurely, hoping that his knowledge that God is not a deceiver will help him further. First, he reasons that he must have a body, as nature teaches that to him more vividly than anything. Further, mind and body are intermingled to form one unit. If the mind were in the body like a sailor in a ship, he would be able to perceive pains and hungers by purely intellectual understanding. Instead, he feels these sensations sharply and directly as if his mind itself were suffering. The confused modes of thinking that arise with respect to these sensations result precisely because the mind and body are intermingled and the mind cannot survey the matter disinterestedly.
Analysis
This section concludes the Meditator's argument by means of the senses for the existence of body. Sensory perceptions must either be created by the Meditator himself, by someone or something else, or by God. The Meditator can rule himself out since he is not aware of creating these perceptions, and they come upon him so forcefully and involuntarily that it would be inconceivable that he could be the creative force behind them. This is proof enough that sensory perceptions have some outside cause. He is naturally inclined to think his sensory perceptions are caused by things that resemble those perceptions. Since God is not a deceiver, he must not be fooling him in giving him this natural inclination. Therefore, he concludes, bodies must be something like what they seem to be. This conclusion will be refined by the distinction between primary and secondary qualities discussed in the next section.
The discussion of sensory perceptions as being "caused" by some outside source marks an important turning point in the history of Western philosophy. The mind is sharply distinguished from the world of bodies around it. The Meditator argues that mind and body have nothing in common, so they must be two totally distinct substances. We could point out that Clark Kent and Superman are very dissimilar and are yet the same thing, and so argue by analogy that mind and body might be two very different ways of looking at the same thing. However, even the primary attributes of mind and body are different. Body is essentially extended, whereas mind is non-extended and essentially thinking. Since the two are totally different, the Meditator concludes that he is only mind, and not body. This is a step beyond what is stated by the sum res cogitans in the Second Meditation, as there the Meditator asserts that he only knows that he is a thinking thing. Now he knows that he is only a thinking thing.
This sharp distinction between mind and body is called "mind-body dualism" and has had tremendous impact on Western philosophy ever since. If sensory experience is in the mind and the bodies that cause our sensations are in the world, the question arises as to how the two can causally interact. What is the connection between mind and world? This has been a great concern in particular for the rationalist philosophers that followed Descartes—Malebranche, Spinoza, and Leibniz being the most important—as well as for philosophy of mind in general ever since. When the mind and the world are held as totally distinct, the mind becomes conceived of as being trapped within the body, unable to know about the world except through a causal interface at the sensory surfaces. As mentioned in the commentary to the Second Meditation, Part 2, the causal interface generated by mind-body dualism has only begun to be questioned in the past hundred years.
