Summary
Part I: Education and the search for a method
Descartes opens by arguing that reason is naturally shared, while differences of opinion arise from the paths along which thought is directed. A strong mind is insufficient without a reliable method.
He reviews his education and values languages, literature, history and mathematics, but becomes dissatisfied with the disputes of philosophy and the uncertain sciences built upon it. He leaves formal study to learn from travel, society and self-examination.
Experience reveals that customs vary widely and that inherited confidence often reflects upbringing rather than truth. Descartes eventually resolves to seek knowledge within himself and to choose deliberately the road his thought will follow.
Part II: The four rules
While wintering in Germany, Descartes reflects upon the greater unity of works designed by one mind. He compares inherited bodies of opinion to irregular towns and decides to rebuild his own beliefs rather than reform society.
He condenses logic, geometry and algebra into four rules: avoid accepting anything not clearly known; divide difficulties; order thought from simple to complex; and conduct complete enumerations and reviews.
Mathematics demonstrates the power of orderly reasoning. Descartes develops a general approach joining geometric representation with algebraic analysis, hoping eventually to address questions beyond mathematics.
He delays a complete attack upon philosophy until his judgement has matured. The method requires not only rules but sustained practice in identifying simple relations and building secure sequences of thought.
Part III: Provisional morality
Rebuilding a house requires temporary accommodation, and theoretical reconstruction similarly requires practical maxims. Descartes first follows the laws, religion and moderate customs of his community while avoiding extreme and irreversible commitments.
Second, he resolves to act firmly once a considered choice has been made. Travellers lost in a forest improve their prospects by maintaining a direction instead of continually changing course.
Third, he seeks to master himself rather than fortune, treating only his own thoughts as fully within his power. Finally, he chooses the occupation of cultivating reason and advancing knowledge through his method.
These rules allow Descartes to travel and observe without losing practical stability. He then withdraws for a period of concentrated inquiry.
Part IV: Doubt, the cogito and God
Descartes rejects beliefs that admit doubt: senses sometimes deceive, people make errors in reasoning, and dreams can resemble waking experience. During this universal doubt, the act of thinking remains undeniable. If he thinks, he exists.
From the cogito he identifies himself essentially as a thinking thing, known independently of bodily existence. He proposes clarity and distinctness as a mark of truth, then asks what guarantees that such perceptions are reliable.
The idea of a being more perfect than himself cannot, he argues, originate solely from an imperfect thinker. God therefore exists and possesses all perfections. Deception would indicate defect, so a perfect God is not a deceiver.
Mathematical and external knowledge can be secured when ideas are clearly perceived. Error arises when thought is confused or when imagination and sense are mistaken for the proper operation of reason.
Part V: Nature, physiology and living beings
Descartes summarises an unpublished account of the physical world. He imagines matter governed by laws established by God and explains how an ordered cosmos could develop from simple mechanical principles.
He considers light, the heavens, terrestrial bodies, plants and animals before concentrating on the heart and circulation. Heat in the heart and the mechanical arrangement of organs explain bodily motion without requiring a separate vegetative soul.
Animal bodies are compared to machines. Human beings differ through rational language and adaptable conduct: no arrangement of organs alone, Descartes claims, can generate indefinitely appropriate speech or universal reasoning.
The rational soul must be specially created and united with the body rather than produced from matter. Descartes also argues that its nature gives grounds for regarding it as immortal.
Part VI: Science, experiment and publication
Descartes initially considers suppressing his work after controversy surrounding another natural philosopher. Yet he believes physical knowledge can replace speculative philosophy with practical understanding and make human beings more capable of improving life, especially health.
Experiments become increasingly necessary as inquiry moves from general principles to particular effects. The same cause may produce several possible outcomes, so observation helps determine which occurs in nature.
Scientific progress exceeds the capacity of one person, suggesting cooperation across generations. Descartes nevertheless doubts that institutional assistance will always be reliable and emphasises the importance of freedom from interruption.
Publication invites criticism that may reveal errors, but responding to every objection can consume the time required for discovery. He explains why he presents selected essays rather than his complete system and why he writes in French for readers using natural reason.
The work closes with Descartes committing his remaining life to useful knowledge of nature while preferring intellectual independence over honours and public office.