Descartes believed it was necessary to take the entire edifice of knowledge down to its foundations to remove existing error. His method was not to doubt everything for the sake of skepticism itself, but to doubt everything that could be doubted, so that one would be left only with what was certain. He began with the usual arguments about the errors of the senses: for instance, the observation that far away objects look smaller than they are.

He then questioned whether he could be sure that there was a world outside of his mind and noted that the insanity of that line of questioning was not unusual if one takes into account the fact that every night, during sleep, there are bizarre distortions in dreams. This raises the question of what exactly is the difference between being awake and being asleep. Descartes notes that there is nothing in the quality of either experience that guarantees which state one is in.

Descartes’ project of doubt next addresses mathematical and logical thinking. Descartes said that our confidence in these processes depends on our confidence that there is a benevolent God who guarantees that what seems self-evident to us really is true, and who guarantees the accuracy of the memory of those past thought processes that are necessary to proceed to a conclusion in a chain of reasoning.

Then, Descartes advances to his most devastating level of doubt: what if there is not a benevolent, all-powerful God, but an evil demon, who instead of supporting our true mental processes, is in fact constantly deceiving us about the workings of our own minds? So now Descartes has raised doubt to the level of doubting the existence of a good and powerful God, which he himself regards as a very disturbing and distressing predicament.