The book is made up of six meditations, in which Descartes first discards all belief in things that are not absolutely certain, and then tries to establish what can be known for sure. He wrote the meditations as if he had meditated for six days: each meditation refers to the last one as "yesterday". The Meditations consist of the presentation of Descartes' metaphysical system in its most detailed level and in the expanding of Descartes' philosophical system, which he first introduced in the fourth part of his Discourse on Method (1637).