One of Descartes' main ideas was that if you think, you exist and whatever you think exists.
Descartes also believed in dualism. Dualism is the idea that there are two worlds: Mind and Matter. He believed that all things fell under one of these two categories.
Dualism also relates to Determinism vs. Free Will. The world of Matter corresponds to the idea of determinism because matter can be thought of as being predetermined and always has a cause and an effect. The world of ideas relates to free will because ideas cannot really be determined.