Leibniz, Gottfried Wilhelm (1646-1716), German rationalist philosopher, mathematician, and logician. He argued that the world is composed of single units (monads), each of which is self-contained but acts in harmony with every other, as ordained by God, and so this world is the best of all possible worlds. Leibniz also made the important distinction between necessary and contingent truths and devised a method of calculus independently of Newton.