Newton's Laws of Motion

Galileo's observation that without friction a body would tend to move forever challenged Aristotle's notion that the natural state of motion on Earth was one of rest. Galileo deduced that it was a property of matter to maintain its state of motion, a property he called inertia. Newton, grasping the meaning of inertia and recognizing that Aristotle's reference to what keeps a body in motion (outside influence) really should have been what changes a body's state of motion, set forth a first law of motion which states: A body at rest remains at rest or a body in constant motion remains in constant motion along a straight line unless acted on by an external influence, called force.