"Rationalism" was a theory of knowledge based on reason. Rationalism asserted that an individual can come to know truth by reason alone and that the human mind is capable of coming to know truth. Empiricism arose out of the Renaissance and the Reformation. Essentially Rationalism was a restoration of Platonism. At the risk of oversimplification, Rationalism was a movement located on the European Continent and the notable personalities previously examined as representatives of the movement include Rene Descartes, Baruch Spinoza and Gottfried Wilhelm Leibniz.
On the other hand, "empiricism" is a theory of knowledge based on sensory perception. Empiricism asserts that individuals have no innate ideas at all other than those derived from experience, which comes to us via our senses. According to empiricism, statements can be known as true and false only by testing them in experience. Empiricism rose to prominence during the 17th and 18th centuries. Notable personalities associated with Empiricism include John Locke, George Berkeley, and David Hume. Whereas Rationalism was a Continental movement, Empiricism is generally regarded as a British movement.
Like Plato, Aristotle says that a thing or a substance is what it is and is known to be that thing in virtue of its "nature" or "essence". This is what Plato calls a "Form." But Aristotle argues that, rather than thinking that the things we ordinarily experience are imitations or copies of what is really real and which exists in the world of Forms, we should think that the things we normally experience in the world are themselves ultimately real. Out knowledge of the world is therefore ultimately based on our experience and not reason alone.
For Aristotle, the essence of a thing, such as a dog's "dogness", is what makes it to be that kind of thing. Dogness is found in all dogs, as opposed to the perspective of Plato who proposed that dogness is found in the world of Forms, and according to Aristotle this can be called a "universal." Universals do not exist apart from actual things in the world, again in contradistinction to Plato and his concept of the world of Forms. Aristotle further proposed that things could be known only by experiencing actual individual things. If all dogs were to die, the universal would no longer exist. It is in virtue of the universal that we now about dogs by knowing them in terms of their essence. But that knowledge is available only when we generalize or "abstract" the universal from our experience.
Aristotle's emphasis on using experience as the basis for knowledge is typical of an empiricist epistemology. For that reason, Aristotle may be regarded as the foundation for British Empiricism.
