René Descartes opens by observing that good sense is naturally distributed equally among all humans, and that our disagreements stem from applying our thoughts along different paths rather than possessing unequal reason. Dissatisfied with the uncertainty and contradictions prevalent in traditional scholastic philosophy and the sciences, he resolves to abandon received opinions and build knowledge from a secure foundation. To guide his search, he formulates four strict rules: never to accept anything as true that is not clearly recognized as such, to divide complex difficulties into manageable parts, to progress methodically from the simplest objects to the most complex, and to ensure complete and comprehensive enumerations.
While undertaking this systematic overhaul of his beliefs, Descartes establishes a provisional moral code to govern his life, emphasizing obedience to country laws, steadfastness in actions once chosen, and the mastery of one's own desires rather than external fortune. Having cleared away inherited prejudices, he applies his method to metaphysics in the fourth part of the discourse. By doubting every sensation and perception, he arrives at his foundational certainty: the act of doubting proves the existence of the thinker. This realization leads to the famous proposition of consciousness, confirming that the essence of the mind is thought, entirely distinct from the material body.
Building upon this certainty of a thinking self, Descartes examines the origin of the idea of perfection. He deduces that the notion of a more perfect being must originate from an entity that actually possesses all perfections, thereby establishing the existence of God. From the veracity of a perfect God, he validates the reliability of clear and distinct perceptions, asserting that our reason, rather than our senses or imagination, is the sole arbiter of truth. This metaphysical groundwork allows him to examine the physical world with renewed confidence in scientific demonstration.
In the subsequent sections, Descartes outlines his physical investigations, detailing the formation of celestial bodies and terrestrial elements through mechanical laws. He provides a renowned explanation for the circulation of the blood and the motion of the heart, comparing biological processes to complex machinery while strictly differentiating conscious human reason from the automatic functioning of animals. He concludes by weighing the reasons for publishing his scientific discoveries against the desire for tranquil study, ultimately deciding to share specific specimens of his method to advance the collective pursuit of natural knowledge and human health.
