The third meditation presents Descartes’ new theory of ideas, to set up his arguments for the existence of his deity.
He considers all he knows at this point – that he is a thinking thing – and notices that this idea is so clear, and distinct from other ideas, that he cannot but assent to its truth. He begins to consider other ideas that meet these criteria; notices that his idea of god fits this, and decides to inquire if this is possible. However, there are important points here that need to be made clear.
He posits three kinds of ideas; innate, adventitious, and imagined. Innate ideas are those wholly from within, while adventitious ones are from without. Imagined ones are created from a combination of other ideas.
Descartes’ posits that innate ideas are such things as mathematics, self, and god. These things are all known a priori, and can be proven in geometric fashion.
Adventitious ideas come from the senses, and are therefore considerably less clear and distinct. These ideas require concepts from meditation four to be considered before assent. These ideas would include such things as bodies, objects, etc.
Imagined ideas are simply that: Imagined. They come from the blending of multiple ideas to create something new. In this category are fictitious creatures like unicorns and satyrs.
Also as part of his theory of ideas, Descartes’ needs to highlight the ‘degrees of reality’ which an idea has. Ideas have degrees of objective reality based on the contents of the idea, while objects have degrees of formal reality. Descartes needs this distinction to show how our innate ideas relate to the world. According to Descartes, any object in the world has as much formal reality as the ideas objective reality, and vice versa.
So Descartes’ not only provides a new concept in ideas, but applies a distinction to their reality. From this base, he attempts to prove the existence of the Judaeo-Christian god.