公理，即運動的定律
Isaac Newton · Philosophiæ Naturalis Principia Mathematica · 1687 · Axioms, or Laws of Motion (Motte trans., 1729)
Law I
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air.
Law II
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively.
Law III
To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone.
論萬有引力（第三卷）
Book III · Of the System of the World · Propositions on gravity
There is a power of gravity tending to all bodies, proportional to the several quantities of matter which they contain.
That all the planets gravitate towards one another we have proved before; as well as that the force of gravity towards every one of them, considered apart, is reciprocally as the square of the distance of places from the centre of the planet.
And this is the force by which the moon is retained in its orbit, and by which bodies fall toward the earth. The force which retains the celestial bodies in their orbits is the very same force we commonly call gravity; for the moon, were it deprived of all motion, would, by its gravity, fall toward the earth.
And therefore the force by which the moon is retained in its orbit is that very same force which we commonly call gravity.
宇宙體系
From “A Treatise of the System of the World” · Newton's own thought experiment
The greater the velocity with which a body is projected, the farther it goes before it falls to the earth. We may therefore suppose the velocity to be so increased, that it would describe an arc of 1, 2, 5, 10, 100, 1000 miles before it arrived at the earth, till at last, exceeding the limits of the earth, it should pass quite by without touching it.
A body projected from the top of a high mountain, parallel to the horizon, with a sufficient velocity, would not fall to the earth at all, but go forward into the celestial spaces, and proceed in its motion in infinitum. And the same reasoning that holds for the projectile holds also for the moon, which is perpetually drawn off from a rectilinear course toward the earth, and made to revolve in a curve.
And by the same principle the planets are kept in their orbits about the sun, and the satellites about their primary planets — the one force of gravity governing them all.
總釋
The General Scholium · added to the 2nd edition, 1713
Hitherto we have explained the phenomena of the heavens and of our sea by the power of gravity, but have not yet assigned the cause of this power. This is certain, that it must proceed from a cause that penetrates to the very centres of the sun and planets, and operates according to the quantity of the solid matter which they contain, and propagates its virtue on all sides to immense distances, decreasing always as the inverse square of the distances.
I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses. … It is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies, and of our sea.
“Hypotheses non fingo.” — General Scholium