The aim of this meditation is to introduce doubt as the method of achieving indubitable certainty regarding the possibility of discovering what is true in the world. Descartes says we must begin by doubting our senses and any knowledge we take to be true that we have acquired from sensuous experience. We must do so because the senses are as reliable as they are deceiving, and if what Descartes is after is truth—defined as knowledge of something that is beyond all question and doubt—then the senses fail to serve as a guarantee in this attempt to discover what is absolutely true, beyond all doubt.

Hence, Descartes writes:

Whatever I have up till now accepted as most true I have acquired either from the senses or through the sense. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once (95).

However, says Descartes, to deny such basic and self-evident facts such as his holding the piece of paper in his hands, his sitting in a room by a fire and wearing winter clothes, and so on, would orient this search for indubitable truth in the direction of madness: