"Definition IVAn impressed force is an action exerted upon a body, in order to change its state, either of rest, or of uniform motion in a right line. This force consists in the action, only, and remains no longer in the body when the action is over. For a body maintains every new state it acquires, by its inertia only. But impressed forces are of different origin, as from percussion, from pressure, from centripetal force.

"In the publication of this work, Edmond Halley, a man of the greatest intelligence and of universal learning, was of tremendous assistance, not only did he correct the typographical errors and see to the making of the woodcuts, but it was he who started me off on the road to this publication. For when he had obtained my demonstration of the shape of the celestial orbits, he never stopped asking me to communicate it to the Royal Society, whose subsequent encouragement and kind patronage made me begin to think about publishing it.

"Isaac Newton's Principia Mathematica is indisputably one of the important and influential books ever written, yet it is scarcely read. Latin is not the problem, for translations in English and French have done little to expand its readership. Those who have attempted to read the Principia quickly recognize the problem: The geometrical style of mathematics is almost opaque to the modern reader.