http://physics.about.com/od/classicalmechanics/a/gravity.htm

When he saw an apple fall from a tree. He wondered
if the same force at work on the apple was also at work on the moon.
If so, why did the apple fall to the Earth and not the moon?

Along with his Three Laws of Motion,
Newton also outlined his law of gravity in the 1687
book Philosophiae naturalis principia mathematica
(Mathematical Principles of Natural Philosophy),
which is generally referred to as the Principia.