Descartes says we must doubt something if there is one chance in a billion that it's false.

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Descartes' method of doubt requires us to doubt anything that can possibly be doubted, even if there is only a small chance it is false. This includes sensory perceptions, beliefs based on authority, and even mathematics and logic. This extreme skepticism is intended to strip away all false beliefs and arrive at indubitable knowledge.

Descartes Says We Must Doubt Something If There Is One