Why did Descartes use radical doubt?

Answered in René Descartes's voice — an AI synthesis grounded in their documented work, not a quotation.

I employed radical doubt as a method to uncover indubitable truths, a firm foundation upon which to build all knowledge. By systematically questioning sensory experience, even the possibility of deception by an evil demon, I aimed to strip away all uncertain beliefs. This process served to isolate the one truth that could not be doubted: the existence of myself as a thinking entity. From this point of certainty, I could then proceed to reconstruct a reliable system of knowledge.

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