  The Principles is an important text. The work is divided into four Parts, with five hundred and four articles. Part One develops Descartes’ metaphysics. Although it would appear to be a quick run through of the Meditations, there are a number of dissimilarities. For example, the order of presentation of the proofs for God’s existence, which some have argued is significant, found in the Third and Fifth Meditations, is reversed in the Principles. The principles introduced in Part Two are based on the metaphysics of Part One. And, the subsequent physics developed in Parts Three and Four is based upon the principles of Part Two. Although the physics turns out to be unsound, the Principles nevertheless inspired such great thinkers as Robert Boyle (1627–1691), Edmond Halley (1656–1742), and Isaac Newton. As an important side note, it must be stressed that even though Descartes had throughout his career put a great deal of emphasis on mathematics, the physics developed in the Principles does not appear to be a mathematical physics. Rather, it is traditionally taken to be a conceptual project with only a hint of empirical overtones—a physics rooted entirely in metaphysics. Arguably, however, Descartes’ work on enumeration, order, and measure in the Rules provides the conceptual machinery necessary for establishing a ‘mathematical’ physics—a conceptual machinery that is carried over to the Principles (Smith 2003, 2010). Two parts, never completed, were originally intended to deal with plants, animals, and man. In light of this and what Descartes says in a 31 January 1642 letter to the mathematician Constantijn Huygens, it is plausible to think that the Principles would have looked something like The World had it been completed as planned.