What role did clear and distinct ideas play in Descartes’s philosophy?

In the philosophy of René Descartes, clear and distinct ideas serve as the fundamental criterion for truth and the foundation of his rationalist system.12

Descartes employed methodic doubt to reject any belief that could be questioned, eventually arriving at the cogito—the intuition that "I think, I am"—which he recognized as indubitable due to the clarity and distinctness with which it presented itself to his reason. He argued that if this intuition is certain, then any other idea perceived with equal clarity and distinctness must also be true.12

These ideas are not sensory perceptions but intellectual conceptions grasped by the mind, such as mathematical truths. Descartes used these ideas to establish the existence of God and the material world, reasoning that a perfect God would not allow him to be deceived regarding what he clearly and distinctly perceives. This reliance on God to guarantee the truth of his perceptions led to the Cartesian Circle, a critique suggesting his reasoning is circular because the validity of his clear and distinct ideas depends on God, whose existence he proves using those same ideas.231