The laws of motion, according to Galileo and Newton.
(1) If no force acts on a body in motion, it will continue to move uniformly in a straight line.
(2) If force acts on a body, it will produce a change of motion proportionate to the force, and in the same direction (as that in which the force acts).
(3) When one body exerts force on another, that other body reacts on it with equal force.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894