The Mathematical Principles of Natural Philosophy (1729)/Preface to Book 3
n the preceding books I have laid down the principles of philosophy; principles, not philosophical, but mathematical; such, to wit, as we may build our reasonings upon in philosophical enquiries. These principles are, the laws and conditions of certain motions, and powers or forces, which chiefly have respect to philosophy. But lest they should nave appeared of themselves dry and barren, I have illustrated them here and there, with some philosophical scholiums, giving an account of such things, as are of more general nature, and which philosophy seems chiefly to be founded on; such as the density and the resistance of bodies, spaces void of all bodies, and the motion of light and sounds. It remains, that from the same principles, I now demonstrate the frame of the System of the the World. Upon this subject, I had indeed compos'd the third book in a popular method, that it might be read by many. But afterwards considering that such had not sufficiently ente'd into the principles, could not easily discern the strength of the consequences, nor lay aside the prejudices to which they had been many years accustomed; therefore to prevent the disputes which might be rais'd upon such accounts, I chose to to reduce the substance or that book into the form of propositions (in the mathematical way) which mould be read by those only, who had first made themselves masters of the principles eltablish'd in the preceding books. Not that I would advise any one to the previous study of every proposition of those books. For they abound with such as might cost too much time, even to readers of good mathematical learning. It is enough if one carefully reads the definitions, the laws of motion, and the first three sections of the first book. He may then pass on to this book, of the System of the World, and consult such of the remaining prepositions of the first two books, as the references in this, and his occasions, shall require.