One who thinks that all human knowledge is gained through experience. This bit of
epistemology is known, believe it or not, as
Empiricism.
John Locke,
David Hume, and
George Berkeley* were empiricists.
John Stuart Mill was even more of one, as he did not even believe that the rules of
math and
logic could be known
a priori.
See also Logical Positivism, Positivism, Verification Criteria of Meaning
If all of this sounds terribly wrong to you, you might be a Rationalist.
If those weren't enough for you, here's a list of some other famous empiricists.
Willard van Orman Quine
William James
Bertrand Russell
Clarence Irving Lewis
Rudolf Carnap
A.J. Ayer (Alfred Jules Ayer)