Discourse on the Method
The problem arises from the fact that the passions, inextricably based in human nature, threaten the supremacy of the thinking subject on which Descartes based his philosophical system, notably in "Discourse on the Method". Descartes had made the thinking subject the foundation of objective certainty in his famous statement, “I think, therefore I am.” It was on this system that he based the possibility of knowing and understanding the world. In allowing that the passions could disrupt the process of reasoning within a human, he allowed for an inherent flaw in this proof. And if man was forced to doubt the truth of his own perceptions, on what could he base his understanding of the natural world?