In this thesis, one of the key topics of Descartes' philosophical thought – his cognitive theory– is critically addressed. First, we are introduced to Descartes' scientific method and its rules. We then follow his methodic doubt, which extends from sensory reality to mathematical ideas. Furthermore, we observe how he arrives at the first certain truth – the truth about one's ownexistence, and then to other certain truths, such as the one about the existence of Godandtheexistence of the material world – and point to the central problem of Descartes' philosophy–the problem of the relationship between mind and reality, that is, the separation of thought andbeing. Finally, we present Descartes' theory about the influence of free will on our knowledgeand explain the reasons why, according to Descartes, we fall into delusions. The philosophical analysis of Descartes' fundamental texts – Meditations on First Philosophy, DiscourseonMethod, Principles of Philosophy, and Passions of the Soul – was used in writing this thesis, and, as an aid in the analysis of these texts, relevant secondary literature and the philosophical encyclopedia Stanford Encyclopedia of Philosophy were used.