In the preceding books I have laid down the principles of philosophy;
principles not philosophical but mathematical: such, namely, as we may
build our reasonings upon in philosophical inquiries. These principles
are the laws and conditions of certain motions, and powers or forces, which
chiefly have respect to philosophy; but, lest they should have 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 World. Upon this subject I had,
indeed, composed the third Book in a popular method, that it might be read
by many; but afterwards, considering that such as had not sufficiently en-
entered into the principles could not easily discern the strength of the conse-
consequences, nor lay aside the prejudices to which they had been many years
accustomed, therefore, to prevent the disputes which might be raised upon
such accounts, I chose to reduce the substance of this Book into the form of
Propositions (in the mathematical way), which should be read by those
only who had first made themselves masters of the principles established in
the preceding Books: not that I would advise anyone 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, and
consult such of the remaining Propositions of the first two Books, as the
references in this, and his occasions, shall require.