Descartes bases his existence on the light of nature by arguing that he has a clear and distinct image of himself. Anything he derives as clear and distinct he considers to be true. He uses the principle of non-contradiction to prove this by comparing examples and finding one false. He cannot be present and not present at the same time. This points to his mathematical reasoning in his approach to all problems and lays the ground work for the deductions he makes about God and the relationship this has on what he clearly understands and what he does not.

Descartes thus reasons that understanding the teachings of nature are less clear than the revealations made by the light of nature since they can be contradicted. He considers anything to do with his senses as teachings of nature and deduces that his senses can deceive him. He examines his experience with senses and intuitive instincts, notes they have failed him on more than one occassion, and thus develops permanent doubt on their reliance. A piece of metal you see sitting in the sun may not look hot but when you touch it you find you were deceived and get burned. This error Descartes would view as being drawn from a judgement where you were misled by nature.

Judgements always have the probability of being false thereby proving you wrong and resulting in error. Teachings by the light of nature, however, are error proof and cannot deceive. Descartes adamantly argues that when the light of nature reveals something to him that it is beyond doubt.

He uses this argument to further define his proof of God~s existence. He deduces that his idea of God had to be placed in him by God himself, since there is no being greater who could have done so. Descartes uses the causal principle for this deduction since he derives that something cannot arise from nothing and something that is more real cannot come from something less real. Thus, he exists so God has to exist. Also, the fact that his idea of God is clear and distinct proves that he is not just sensing his existence but that this revelation come from the light of nature, which cannot deceive. God being perfect cannot deceive so his idea planted in Descarte has to be true.