The book sets forth the principles of nature—the Laws of Physics—as Descartes viewed them. Most notably, it set forth the principle that in the absence of external forces, an object's motion will be uniform and in a straight line. Newton borrowed this principle from Descartes and included it in his own Principia; to this day, it is still generally referred to as Newton's First Law of Motion.[2] The book was primarily intended to replace the Aristotelian curriculum then used in French and British universities. The work provides a systematic statement of his metaphysics and natural philosophy, and represents the first truly comprehensive, mechanistic account of the universe.
Preface to the French edition
Descartes asked Abbot Claude Picot to translate his Latin Principia Philosophiae into French. For this edition, he wrote a preface disguised as a letter to the translator, whose title is "Letter of the author to the translator of the book, that may be used as a preface." This was published in 1647, a date already in the mature, final period of his life. In this writing, Descartes provides some reflections on his ideas of wisdom and philosophy. Its content may be summarized as follows.[3]
Concept of philosophy
Philosophy is the study of wisdom, understood as the ability to conduct the human activities; and also as the perfect knowledge of all the things that a man can know for the direction of his life, maintenance of his health, and knowledge of the arts. Only God is perfectly wise, and the man is more or less wise, in proportion to the knowledge he has of the most important truths.
The degrees of knowledge
Descartes identifies four degrees of knowledge which he names common, and a fifth degree he designates higher. The first degree consists of clear and evident notions that can be acquired without the need for any meditation. The second degree is all that is learned by means of the senses. The third comprises what we learn when talking with others. The fourth consists of what we can learn from the writings of those capable of giving good instructions.
Higher wisdom
There have been great people throughout history who have pursued a better and more secure wisdom, a fifth degree of knowledge. This has consisted of the search for the first causes, and those that have followed this pursuit have been named philosophers, but he thinks that none have yet been successful.
Doubt and certainty
Since Plato and Aristotle, there has been discussion on doubt and certainty. Those that have favored doubt have arrived at extremes of doubting even the most evident things, and those that have sought certainty have relied excessively on the senses. Though it has been accepted that the senses may mislead us, according to Descartes, nobody had yet expressed that the truth can not be based on the senses, but in the understanding, when it is founded on evident perceptions.
Meditations on first philosophy
The search for the first causes, or basic truths, as undertaken by Descartes is contained in this work. It explains the metaphysical principles on which to build the rest of knowledge.
The tree of philosophy
Descartes describes philosophy as like a tree, whose roots are metaphysics, its trunk physics, and the branches are the rest of the sciences, mainly medicine, mechanics, and morals that is the last level of wisdom. In the same way that trees have fruits in their outer parts, the usefulness of philosophy is also contained in the areas that stem from its foundation.
Copies and modern editions
René Descartes' Principia Philosophiae. This copy was printed in 1656, and is owned by Southwest Research Institute rare book collection at the Tom Slick Memorial Library in San Antonio, Texas.
Reidel, a Dutch publisher, released an English edition of Principia philosophiae in 1983 (ISBN90-277-1451-7), translated by Valentine Rodger and Reese P. Miller with explanatory notes. Though a translation of the original 1644 Latin work, this edition by Rodger and Miller includes additional material from the 1647 French translation.
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