Pure Mathematics
As Descartes writes in the First Meditation and references multiple times throughout the rest of them, he believes that pure mathematics (arithmetic and geometry) are possibly the purest form of scientific knowledge, as they are not dependent upon external experimentation using data from the physical sensual world, but entirely self-contained within abstract theory and reason itself. Pure mathematics, therefore, represents the truth available through the use of pure reason, immutable laws of nature that are impossible to break.
God
God is mentioned many times throughout these Meditations, and Descartes clearly believes in his actual, literal existence, but God nevertheless functions as a sort of symbol in this work. After logically proving the fact that he himself exists (or at least his mind), he immediately attempts to prove and concludes that God exists, claiming that his existence is logically necessary. God comes to represent the order of the universe, along with everything true that it is impossible to imagine otherwise.
Wax
In the Second Meditation, Descartes uses the example of a piece of wax in his study to explore the concept of the physical body. He marvels at his ability to recognize the wax as such, even though it morphs from one form into a completely separate one when he holds it next to the fire, thus concluding that his recognition of the wax as wax is dependent on his mind, not his sense. The piece of wax subsequently comes to represent, in essence, all physical bodies.
The River and the Banks
In the Fifth Meditation, Descartes uses a symbolic analogy to describe his belief in God: trying to believe in existence without God is trying to believe in a river without banks. It's hypothetically possible that neither a river nor banks actually exist anywhere in the world, but the point is that the two are inextricably intertwined, and one cannot exist without the other. The choice of a river is also symbolic, given the Christian imagery of Christ as living water, as well as the prominent usage of river imagery in the Bible.
Highlands Without Lowlands
Also in the Fifth Meditation, Descartes uses another symbolic analogy to describe his belief in God: believing in existence without God is like believing in lowlands without highlands. It's a logical impossibility; one only exists due to the existence of the other. This mutual causation highlights what is implied in the river/banks metaphor: God and existence are concepts that must exist alongside each other or not at all, at least in Descartes's mind.
