Any theory emphasizing sense-experience (including introspection) rather than reason or intuition as the basis for either some or all of our knowledge; 'basis' referring usually to justification, though sometimes to psychological origin.
Empiricism can concern either propositions or concepts, rejecting (most) a priori ones; for John Locke (1632-1704) all concepts ('ideas') were empirical, but propositions connecting them could be known a priori. Extreme empiricists confine our knowledge to immediate experience (and introspection) and fall into the egocentric predicament.
Less extreme ones allow other knowledge to be reached from such experience, or confine their empiricism to certain spheres. Empiricists usually allow propositions of mathematics and logic to be a priori, but insist that they are analytic (that is, roughly, reducible to tautologies) rather than synthetic (embodying substantive knowledge).
Also see: rationalism
D Odegard, 'Locke as an Empiricist', Philosophy (1965)