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Sir I s a a c N e w t o n’s Book I.

motions are regulated; in the other, his Optics, he discoursesof the nature of light and colours, and of the action betweenlight and bodies. This second treatise is wholly confined tothe subject of light: except some conjectures proposed at theend concerning other parts of nature, which lie hitherto moreconcealed. In the other treatise our author was obliged tosmooth the way to his principal intention, by explaining ma-ny things of a more general nature: for even some of the mostsimple properties of matter were scarce well established at thattime. We may therefore reduce Sir I s a a c N e w t o n’s do-ctrine under three general heads; and I shall accordingly di-vide my account into three books. In the first I shall speakof what he has delivered concerning the motion of bodies,without regard to any particular system of matter ; in the se-cond I shall treat of the heavenly motions; and the third,shall be employed upon light.

2. In the first part of my design, we must begin with anaccount of the general laws of motion.

3. These laws are some universal affections and proper-ties of matter drawn from experience, which are made useof as axioms and evident principles in all our arguings upon themotion of bodies. For as it is the custom of geometers to -assume in their demonstrations some propositions, withoutexhibiting the proof of them ; so in philosophy, all our rea-soning must be built upon some properties of matter, first ad-.mitted as principles whereon to argue. In geometry these ax-ioms are thus assumed, on account of their being so evident

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