Newton's laws of motion (law of inertia, fundamental law of dynamics, law of reciprocal actions) were verified by experiment and observation for over 200 years, and they are excellent approximations at the scales and speeds of everyday life. At the atomic scale, they become a poorer approximation to quantum mechanics, and at speeds comparable to the speed of light, they become a poorer approximation to relativity. Just as they fail for material objects moving at speeds close to the speed of light, they fail for light itself.
Newton's first law appeared to be in the past just a special case of the second law, and it was thought Newton stated the first law separately simply in order to throw down the gauntlet to the Aristotelians. However, modern physicists think that the First Law defines the reference frames in which the other two laws are valid. These reference frames are called inertial reference frames or Galilean reference frames, and are moving at constant velocity, that is to say, without acceleration. (Note that an object may have a constant speed through its motion path and yet have a non-zero acceleration, as in the case of uniform circular motion. This means that the surface of the Earth is not an inertial reference frame, since the Earth is rotating on its axis and orbits around the Sun. However, for many experiments, the Earth's surface can safely be assumed to be inertial.
The law of gravity became Sir Isaac Newton's best-known discovery. Newton warned against using it to view the universe as a mere machine, like a great clock.