146 Sir I S A A C N E W T O NTs Book II.

actions of whatever agents there may be in them. We musttherefore allow, that in the attra&ing and repelling powerswhich obtain in nature, from whatever fort of cause they mayarise, aSlion and re-aftion are always equal ; and since thislaw obtains in all forts of motions that arise from impulse, wemay be the more surprized if we should find the philosophersthat explain those powers from impulse call it in question.Even in the motions produced by voluntary and intelligentagents, we find the fame law take place ; for tho’ the principleof motion, in them, be above mechanism, yet the instrumentswhich they are obliged to employ in their actions are so farsubject to it as this law requires. When a person throws astone, for example, in the air, he at the fame time reactsupon the earth with an equal force ; by which means thecentre of gravity of the earth and stone perseveres in the famestate as before. And the necessity of this law, for preservingthe regularity and uniformity of nature, well deserved the at-tention of those who have wrote so sully and usefully of jinalcauses, is they had attended to it,.

C PI A P.