6 ideas
Treat any belief that admits even the slightest ground for doubt as if it were false, and attack whole classes of belief by undermining their source rather than checking claims one by one. A foundation is only secure if it survives the most extreme doubt you can construct, so whatever survives becomes the base for rebuilding everything else.
Descartes raises the doubt in stages: the senses sometimes deceive; there are no sure marks that separate waking from dreaming; and a supremely powerful deceiver could make him err even about arithmetic. Each stage knocks out a wider class of belief, until only what no deceiver could fake is left.
The act of doubting or being deceived requires a doubter, so 'I am, I exist' is necessarily true whenever it is thought or said. This certainty covers only a thinking thing, one that doubts, understands, wills, imagines, and senses. It does not yet cover a body or a world.
A piece of beeswax brought near the fire loses its smell, color, shape, and hardness, yet we judge that it is the same wax. Since every sensed quality changed, the wax's identity must be grasped by the intellect's judgment, not by the senses. Knowing bodies depends more on mind than on perception, and the mind is known better than any body.
A cause must contain at least as much reality as its effect, including the representational content of an idea. A finite, doubting mind could not produce the idea of an infinite, perfect being from its own resources, so such a being must exist as its cause. A perfect God would not be a deceiver, and that guarantees that clear and distinct perceptions are true.
Error arises because the will, which is unlimited, affirms or denies more than the finite intellect clearly perceives. The remedy is to suspend judgment wherever perception is not clear and distinct. On this view, mistakes result from misusing freedom, not from defective faculties.