What is the difference between empiricism and rationalism ?
Empiricism is a school of philosophy which holds that ultimate reality is derived from sense experience. As a philosophy it's closely allied with the methodology of natural science. The only kind of knowledge that matters for the empiricist is that which can be formally measured or verified. Everything that comes to our minds does so through the senses; our whole mental life is therefore derived from our sense experience. As such, we have no innate ideas; our daily experience simply writes itself upon the blank minds with which we are born. This process explains how we derive knowledge of the world outside us. Without sense experience there can be no knowledge.
Rationalism, on the other hand, does believe that there is indeed such a thing as a priori knowledge. That is to say we can have knowledge of certain and indubitable truths from the exercise of our reason, prior to sense experience. A paradigm example of this would be mathematics, where we don't need to rely on our senses to establish that 2+2=4. A priori knowledge is a higher form of knowledge, one that gives us access to a more substantial truth which transcends the everyday world.
For the rationalist, the senses often prove unreliable guides to the truth. For example, we may perceive a straight stick as bent when it is underwater—so, we need the faculty of reason to clarify matters. Reason is everything to rationalists, the ultimate standard of truth against which the world of sense experience is to be measured, and often found wanting.
Rationalist philosophers such as Spinoza constructed elegant, detailed, and elaborate systems of thought based on reason alone. Later empiricist thinkers such as Francis Bacon criticized such systems on the grounds that though they were internally coherent and intellectually rigorous they were entirely self-contained and didn't actually refer to anything "out there" in the real world, the world of objects studied by natural science.
Bacon uses a colorful metaphor to elaborate this point. Empiricists are like bees, in that they take items from the natural world and transform them into something different. Just as bees collect pollen and turn it into honey, empiricists take sense impressions derived from the world around us and turn them into ideas, concepts and hypotheses.
Rationalists, on the other hand, are like spiders. For just as spiders create beautiful, elaborate webs out of their own bodies, so rationalists construct intricate philosophical systems out of their reason. But as such systems are derived entirely from the operation of reason, they cannot tell us much about the empirical world of things and objects. Indeed, rationalistic systems of thought can only really tell us what's going on in the minds of the philosophers holding those ideas.
Empiricism is a theory of knowledge that emphasizes the role of the five senses in obtaining knowledge. That is, empiricists focus on knowledge gained by experience. Sometimes, this is called a posteriori knowledge. We are born as a blank tablet (tabula rasa) on which information is imprinted by sense data. Typically, empiricists do not give any credence to a priori knowledge (knowledge that we are born with or knowledge that exists prior to experience). One problem empiricists face is the role of the mind in receiving this sense data. If the mind is the blank tablet model, then it is a passive function. However, constructive empiricism, one of the different manifestations of this philosophy, holds that the mind can construct knowledge based on, and in addition to, the sense data gained by experience. Famous empiricists include John Locke, George Berkeley, and John Stuart Mill.
Rationalism is a group of views that emphasizes the role of reason, intuition and introspection rather than sensory experience. Rationalism began with the ancient Greeks and the extremism of rationalism was immediately confronted with problems such as language acquisition. A person might have the inherent capability for language but learning a language requires sensory experience. Therefore, rationalists had to admit the role of experience in confluence with mental reason. Descartes' famous “I think, therefore I am” is a rationalist statement but the words in the phrase were obtained from sensory experience. Famous rationalists include Rene Descartes, Immanuel Kant and Baruch Spinoza.
For the “rational” rationalist, the knowledge we obtain through our reason can include concepts obtained from sensory experience but knowledge can also be a priori. There is a biological or genetic ability prior to experience; so we are not born as a blank tablet. We are born with innate structures of knowing, one example being Chomsky's claim of our innate grammar.
Since Descartes, rationalists have had to come up with nuances to the theory in order to address how the mind and external world interact. The same could be said for the more recent philosophy of empiricism.