The Meditator happily concludes that he can know at least that he exists, that he is a thinking thing, that his mind is better known than his body, and that all clear and distinct perceptions come by means of the intellect alone, and not the senses or the imagination.
The first paragraph of the above summary covers the ninth paragraph of the Second Meditation. We could identify this moment as the invention of the modern mind. The Aristotelian conception of the mind separates intellection and understanding as attributes of a soul that survive death. Sensing, imagining, willing, etc., are all attached to the sensory world and are therefore distinct, according to Aristotle. In the Cartesian conception of mind, there is a sharp distinction between mind and world, where all those activities--like sensing and imagining--that could take place in dreams or in disembodied minds are considered mental activities, and exist only in the mind. Things in the world such as trees or light waves are then totally separate from things in the mind, and it becomes a major concern for modern philosophy to determine how the two connect. For instance, there seems to be some connection between my visual sensations and the objects in the world that I see, but since visual sensations are a part of the mind and the objects I see are a part of the world, it is very difficult to determine what that connection is. This picture of mind may seem intuitive to us now, but it and the theories of mind that have sprung from it originate in Descartes. Only in the twentieth century have philosophers like Wittgenstein, William James, and J. L. Austin come to question Descartes' sharp distinction between mind and world.
The rest of the Second Meditation concentrates on the "Wax Argument" with which Descartes hopes to show definitively that we come to know things through the intellect rather than through the senses and that we know the mind better than anything else. His argument focuses on the process of change by which solid wax melts into a liquid puddle. The senses seem to tell us things about the world, and Descartes admits that what we know about the solid piece of wax we know through the senses. The senses can similarly inform us about the melted wax, but they cannot tell us that the melted wax and the solid wax are one and the same. Nor, Descartes argues, can the imagination. Only the intellect can organize and make sense of what we perceive. The senses only perceive a disconnected jumble of information: the intellect is what helps us to understand it.
This argument is another move against the Aristotelian theory of knowledge, according to which all knowledge comes from the senses. Descartes acknowledges that the senses inform us about the world, but asserts that the senses can only give us disorganized information. Without the intellect, we could make no sense of what we perceive. Descartes thus places himself firmly in the rationalist camp, as compared to empiricists such as Aristotle or Locke who argue for a sense-based theory of knowledge.
Descartes' next move is a little more questionable. He asserts that "I" cannot know with certainty that what "I" perceive is real (as per the doubts of the First Meditation), but that sensory perception, as a form of thought, confirms that "I" exist ("I" being the mind.) Every time "I" perceive "I" am thinking, and in thinking "I" am enacting the cogito. Every perception confirms the existence of "my" mind and only gives dubitable evidence for the existence of the world. Thus, Descartes concludes, the mind is better known than the body.
This argument is plausible if Descartes means that the existence of the mind is better known than the existence of the body, but it seems that he wants to say that the nature of the mind is better known than the nature of the body. That is, Descartes wants to say that "I" know not only that the mind exists, but also "I" know more about the mind than about the world outside the mind. This argument would only hold if every thought, perception, imagination, etc., told "me" something new about the mind. But, according to the cogito, all these thoughts tell "me" only one and the same thing: that "I" exist, and that "I" am a thing that thinks. Descartes is not as clear as we might like him to be as to what and how exactly each new thought makes the mind better known than the body.