Gottfried Wilhem Leibniz (1646-1716) was a German philosopher and polymath (someone with expertise in many different areas). He is considered one of the strongest proponents of rationalism, a school of philosophy which stresses that knowledge is accumulated primarily, solely through thought.
Rationalists did not believe that authentic knowledge could be gained via the senses, through empiricism (experience). Cogito ergo sum – I think, therefore I am – as Descartes put it, the founder of modern rationalism.
The historians of philosophy contrast rationalism with British empiricism, led by David Hume, Thomas Hobbes and John Locke. These empiricists argued that knowledge is gained first and foremostly via the senses. Simply stated, experience is more important than (informs) pure thought.
Although such overly simplified characterizations are questioned by today’s experts, they show a fundamental difference between continental philosophy (German and French) on the one side and British, and later British-American, philosophy on the other.
The competition between rationalism and empiricism is in the end a battle between deduction and induction.