How the Methodic Doubt works.

In order to determine whether there is anything we can know with certainty, Descartes says that we first have to doubt everything we know.

 Since sense experience is sometimes deceiving, it is obvious to Descartes that "a posteriori" claims cannot be the basis for claims of knowledge. "A posteriori" means knowledge you gained from actual experience, like saying that the milk in the fridge is sour because you tasted it and it was sour.

Descartes says that in order to know if something is true we must first doubt everything we know.
When we dream we imagine things happening often with the same sense of reality as we do when we are supposedly awake. Just as a person who has an amputated limb has real sensations and feels real pains in a hand or a foot that no longer exists, we sense that we have a body and interact with other bodies. This means that we can be fooled by our own body or by our surroundings into thinking something that isn't real or true is real or true.