Rationalism and empiricism are two schools of philosophy which are sometimes opposed to one another. Specifically, the two schools are both related to empiricism, the area of philosophy that addresses the nature of knowledge. Rationalism and empiricism provide two different accounts of how knowledge is to be acquired. Each school of thought has its own advantages and disadvantages.
Empiricist philosophers believe that knowledge derives solely from the senses. In contrast with some other schools of philosophy, empiricists deny that humans are born with knowledge. Instead, they believe that humans develop their thoughts and beliefs based on what they perceive -- and that to be true, knowledge must be verified by observation. Famous empiricist philosophers include William of Ockham, Thomas Hobbes, Francis Bacon, John Locke and John Stuart Mill, as well as ancient philosophers such as Aristotle.
In contrast to empiricism's emphasis on observation as the only valid source of knowledge, rationalism argues that knowledge can be obtained by the pure application of reason without outside stimulus. Some rationalist philosophers emphasis pure reason over other forms of inquiry, while others claim that reason is in fact the only means of achieving truth. Many rationalist philosophers have also been fascinated by mathematics, a field which seems to offer truth without real-world observation. Famous rationalists have included Rene Descartes, Baruch Spinoza and Gottfried Leibniz.
Both rationalism and empiricism have advantages as forms of epistemology. Rationalism has a stronger claim to address knowledge outside the realm of human experience. For instance, Descartes' famous claim "I think, therefore I am" is what is known as an "a priori" statement -- it proceeds from purely internal reason rather than outside observation. Rationalism therefore applies well to fields of knowledge that have no basis in evidence, such as mathematics. For instance, mathematics requires its practitioners to consider figures such as perfect circles, which are never observed in reality. By contrast, empiricism forms the philosophical basis for scientific inquiry.
Both empiricism and rationalism have limitations as philosophies of knowledge. Empiricism depends on the validity of sensory input; where human perception is unreliable, empiricism struggles to offer any foundation for knowledge. Even prominent rationalist philosophers like Gottfried Leibniz admitted that they used an empirical basis for most action and decision making, without abstract theory behind it. In fact, the distinction between rationalism and empiricism is a modern one; most empiricist philosophers believed that abstract thought was important, while rationalists admitted the important place of empiricism in human knowledge.