The Meditator then contrasts his natural assumption that adventitious ideas represent outside objects with his knowledge that he exists. He cannot doubt that he exists or that this fact follows from the fact that he doubts, because that truth is "revealed...by the natural light." Natural assumptions, on the other hand, are far less certain than the natural light, and have misled him in the past. Further, he has no reason to suppose that these ideas are adventitious at all. The will may have no effect on them, but they still may be produced from within him. And if they do come from without, there is no reason to think that they resemble the objects that they represent. For instance, the sun looks very small according to our senses, but astronomical reasoning suggests that it is in fact very large.
Having ascertained that he exists and that he is a thinking thing, the Meditator tries to determine how he can know these things, and whether he might come to know other things as well by similar means. He concludes that his knowledge of the cogito and the sum res cogitans are clear and distinct perceptions. Thus, he concludes, all clear and distinct perceptions (which he sometimes refers to as "the natural light") must be certain.
The reasoning here might seem a little circular. On one hand, the cogito is certain because it is clearly and distinctly perceived. On the other hand, clear and distinct perceptions must be certain because they are the means by which the certainty of the cogito is achieved. There is also the difficulty raised with the case of geometry and arithmetic. These truths seem clear and distinct to us as well, but there is still the possibility that we are deceived with respect to them. And if God can deceive us of our clear and distinct perceptions, perhaps even the cogito can be cast back into doubt.
Descartes seems to want to escape the problems involved in clear and distinct perceptions by relying on God's existence to make them true. However, Descartes also seems to want to prove God's existence by claiming it as a clear and distinct perception. This further conundrum is famously called the "Cartesian Circle," and we will look at it more closely in the commentary to the Third Meditation, Part 3.
The discussion of the theory of ideas is a preamble to Descartes' attempt to prove the existence of God. According to Descartes, ideas are the atoms of thought, and all thought is made up of composite ideas. Descartes' suggestion that ideas are "as it were the images of things" is not meant to reduce ideas to being simply visual representations. We can have ideas of God, of justice, of how to fix the kitchen sink, none of which are necessarily accompanied by an image--hence the "as it were" that qualifies "the images of things."
Some ideas are ideas in the strict sense only, while others are ideas in the strict sense as well as something else. That "something else" can be volition, emotion, or judgment. Descartes is particularly interested in judgments, since these are the things we can be wrong about, and he wishes to identify the source of error in order to identify the source of doubt. Most error in judgment has to do with identifying things in the material world, since that is where the mind tries to pass judgment regarding things outside of it. Thus, of innate, invented, and adventitious ideas, Descartes takes the greatest interest in adventitious (not inherent but added extrinsically) ideas. He realizes that often we assume we are perceiving things outside our mind without any degree of certainty or justification.