Descartes wrote his first work Compendium Musicae, a small treatise on music in 1618.
RULES FOR THE DIRECTION OF MIND
Descartes composed an incomplete and roughly drafted treatise on methodology, Rules for the Direction of Mind, in 1629. Possibly never intended for publication, it was published posthumously in 1701.
In this book he rejects the scholastic view that there are as many kinds of knowledge as knowable objects. He asserts that all knowledge is of one kind only since it can be acquired only by using the human mind. He rejects the syllogistic reasoning of scholastic philosophy as useless for the discovery of truth; in its place he proposes the analytic mode of reasoning.
Descartes started working on The World in 1629 and almost completed the draft by 1633, but he abandoned plans of having it published when he heard of the condemnation of Galileo by the Catholic Church for his advocacy of the Copernican theory. His book also endorsed this theory of earth's movement around the sun. Only a few fragments of this book could be found, and these were published after his death.
In this book, Descartes planned to unite all his scientific results to give a comprehensive physical theory of the Universe. This scientific work was a set of interconnected propositions based on a unitary strand of premises and argumentation.
DISCOURSE ON METHOD
Descartes published anonymously his most famous work Discourse on Method with three appendices Optics, Meteorology and Geometry in 1637. This book was revolutionary not only in its contents, but also in the way it was written. It was written in French, and not in Latin, in a captivating, first person style.
The first three sections of this book demonstrate the four rules of the Cartesian Method. The fourth section introduces the Cartesian doubt and outlines the metaphysical foundations of Cartesian doctrine. The mind-matter dualism is also introduced here.
The three appendices were added to the Discourse to demonstrate the efficacy of his methodology by applying these principles to three scientific problems. In the first appendix Optics, the laws of refraction are formulated. The second appendix, Meteorology, attempts a scientific explanation of the weather, including that of the rainbow. The last appendix, Geometry, lays the foundation of modern analytic geometry by introducing the Cartesian coordinates and, by using algebraic notation to deal with geometrical problems.
MEDITATIONS ON FIRST PHILOSOPHY
The first edition of Meditations on First Philosophy was published together with the first six sets of Objections and Replies in 1641. The book was in Latin and was dedicated to the Dean of the Sacred Faculty of Theology of Paris i.e. the Sorbonne. Its second edition was published with all the seven sets of Objections and Replies in 1642.
This work is a more of an elaborate development of the metaphysical foundations of Cartesian doctrine. It includes critical opinions of philosophers and theologians such as Hobbes, Gassendi and Arnauld, and replies of Descartes to their objections.
THE PRINCIPLES OF PHILOSOPHY
The most complete statement of Descartes' mature philosophy, The Principles of Philosophy, written in Latin, was published in 1644. It was dedicated to Princess Elizabeth, the daughter of Frederick V against whose army he had fought as a young Bavarian army officer.
The first part of this book explains Cartesian metaphysics, the second details the principles of Cartesian physics, the third explains the Universe using these principles of physics, and the fourth part deals with a variety of terrestrial phenomena. There is a total rejection of spiritual notions in the scientific explanations of the latter three parts.
PASSIONS OF THE SOUL
There was a voluminous correspondence between Princess Elizabeth of Bohemia and Descartes. The questions raised in her letters are dealt with in the book Passions of the Soul published in 1649. This book deals with the problems of the interaction of body and soul. It is a combination of psychology, philosophy and ethics, and contains Descartes' theory of two-way causal interaction via the pineal gland.