In Epistemology, Rationalism is the dogmatic theory which holds that knowledge is acquired through rational, or logical, reflection on innate ideas.
A core tenet of Rationalism is the belief that the senses are unreliable sources of information since they often deceive us. If all our beliefs were acquired through sensation (as Empiricism holds) we would be unable to obtain knowledge. Thus, rationalists believe there must be an alternative source of ideas which is not subject to the fallibility of the senses. If some ideas are innate to the mind (or soul), and if these ideas are self-evidently or analytically true, then they can provide a reliable foundation upon which to validate or justify beliefs acquired through the senses.