The treatise of Rules for the Direction of the Mind can be traced back to the period of Descartes’ youth in which, until rule eight, he has presented some foundations of the theory of truth and error; such as the truth being founded on manifest and distinct intuition and deductive inference which is an adaptation of his analytical geometric theory. However, in rule twelve, he has vaguely addressed how the set of human faculties commits errors. Since this philosopher has also (...) given attention to the matter of errors in Principles of Philosophy and in the treatise Discourse on the Method of Rightly Conducting One’s Reason and of Seeking Truth in the Sciences and Objections and Replies, in this article an attempt has been made to provide a comprehensive view and critique of Descartes’ theory regarding truth and error, albeit, focusing more on rule twelve of the treatise of Rules for the Direction of the Mind. According to Descartes, two things play an essential role in cognition; understanding and will. One of the differences between these two is that the scope of understanding is limited and it cannot acquire knowledge of everything; however, the scope of the will is unlimited. Now, if there were to be errors in our perception, they must either arise from understanding or will.In his treatises, Descartes has addressed the different dimensions of errors in perception and, overall, has shown the various aspects of error in the theory of cognition. The existence of pre-judgments and differences between the levels of understanding and will, that is, the infinity of the will and finiteness of understanding, are some of the matters that Descartes counts as factors of errors. It is noteworthy that while Descartes considers errors to be the result of the interference of the will in matters that the intellect does not have sufficient knowledge of, that is, clear and distinct knowledge, he considers this error to be true in the domain of judgment or assertions. The present article recounts the reasons for errors and analyzes them based on different works of Descartes, including Rules for the Direction of the Mind but also considers Descartes to be unjustified in some matters and criticizes him regarding those.One of the criticisms aimed at Descartes’ theory of truth is that he states that only manifest and distinct perception is true and what lies beyond it is error. He considers this principle to be demonstrable or innate. After studying the different aspects of this theory, the present article shows that even in the area of mathematics to which Descartes has given serious attention and, in fact, according to him, it is a mark of manifest and distinct thought, it is not acceptable to consider this an innate matter unless Descartes accepts it as an axiom of his philosophical system. The absoluteness of the will is also not a sufficient argument regarding the infiniteness of will which provides another possibility for error; however, attention to the essential property of the will which is indefectible is noteworthy. The theory of considering concepts and realities to be graded which has been studied in different parts of his views especially in proving the creator and regarding infinity is also not provable in Descartes’ philosophical system unless he considers it as an axiom. (shrink)