The laws of motion (Mathematical Principles of Natural Philosophy, book I):

• ``Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.''

• ``The change of motion is proportional to the motive force impressed; and it is made in the direction of the right line in which that force is impressed.''

• ``To every action there is always opposed an equal reaction: or the mutual action of two bodies on each other are always equal, and directed to contrary parts.''

The principle of universal gravitation (Mathematical Principles of Natural Philosophy, book III):

• ``There is a power of gravity pertaining to all bodies, proportional to the several quantities of matter which they contain.''