Rene Descartes, in Meditations, describes the reasons why mankind makes false judgements. Given that some of Descartes' writing can be relatively difficult to understand, the use of layman's terms is very appropriate to use in order to understand his ideology.
Descartes believed that mankind would err when they lacked the knowledge needed to make good decisions (regarding both actions and beliefs). Therefore, given lack of knowledge, mankind does not know all they need to know to make solid and educated decisions. Instead, mankind will attempt to infer (using prior knowledge to make an educated guess) what needs to be done. As with inferences, not all are correct. A person's prior knowledge may be skewed, and the person's inference will be wrong.
Descartes, therefore, deemed that errors can be avoided when, and only when, the information presented is clear and distinct (given that this clarity and distinctiveness will bring about no error). This ideology is fully denoted in Descartes' "Method of Doubt."