The Sixth and final Meditation is entitled "The existence of material things, and the real distinction between mind and body," and it opens with the Meditator considering the existence of material things. The Meditator accepts the strong possibility that material objects exist since they are the subject-matter of pure mathematics, the truths of which he perceives clearly and distinctly. He then produces two arguments for the existence of material things, one based on the faculty of the imagination, the other based on the senses.
He first distinguishes between imagination and pure understanding. In the case of a triangle, he can perceive that a triangle is three-sided and derive all sorts of other properties using the understanding alone. He can also perceive these properties with the imagination, by picturing the triangle in his mind's eye. However, the weaknesses of the imagination become clear when he considers a thousand-sided figure. It is very difficult to picture it in his mind's eye, and more difficult still to differentiate that mental image from the mental image of a 999-sided figure. The pure understanding, however, dealing only in mathematical relations, can perceive all the properties of a thousand-sided figure just as easily as it can a triangle.
The imagination cannot be an essential property of his mind, since the Meditator could still exist even if he could not imagine. Therefore, the imagination must rely on something other than the mind for its existence. The Meditator conjectures that the imagination is connected with the body, and thus allows the mind to picture corporeal objects. In understanding, the mind turns inward upon itself, and in imagining, the mind turns outward toward the body. The Meditator admits that this is only a strong conjecture, and not a definitive proof of the existence of body.
The Meditator then turns to reflect on what he perceives by means of the senses. He perceives he has a body that exists in a world, and that this body can experience pleasure, pain, emotion, hunger, etc., and can perceive other bodies with extension, shape, movement, hardness, heat, color, smell, taste, etc. He thinks it not unreasonable to suppose that these perceptions all come from some outside source. They come to him involuntarily, and they are so much more vivid than the perceptions he consciously creates in his own mind. It would be odd to suggest that he can involuntarily create perceptions so much more vivid than the ones he creates voluntarily. And if they come from without, it is only natural to suppose that the source of these sensory ideas in some way resemble the ideas themselves. From this point of view, it is very easy to convince oneself that all knowledge comes from without via the senses.
What Descartes understands by "body" is somewhat counter-intuitive and is closely linked to his physics, which is not made readily apparent in the Meditations. This section of commentary will depart a bit from the text it comments on in order to clarify some concepts of Cartesian physics.
The entirety of Cartesian physics rests on the claim that extension is the primary attribute of body, and that nothing more is needed to explain or understand body. "Extension" means extended in space, and so a body is anything that occupies space. We should recall that Descartes was also a great mathematician, and invented both analytic geometry and the coordinate system that now bears his name. Descartes' physics is highly mathematical, and we should understand bodies as anything that could be graphed in coordinate space.