Defined narrowly, epistemology is the study of knowledge and justified belief. As the study of knowledge, epistemology is concerned with the following questions: What are the necessary and sufficient conditions of knowledge? What are its sources? What is its structure, and what are its limits? As the study of justified belief, epistemology aims to answer questions such as: How we are to understand the concept of justification? What makes justified beliefs justified? Is justification internal or external to one’s own mind?
Evidence vs. Reliability
What makes justified beliefs justified? According to evidentialists, it is the possession of evidence. What is it, though, to possess evidence for believing that p? Some evidentialists would say it is to be in a mental state that represents p as being true. For example, if the coffee in your cup tastes sweet to you, then you have evidence for believing that the coffee is sweet. If you feel a throbbing pain in your head, you have evidence for believing that you have a headache. If you have a memory of having had cereal for breakfast, then you have evidence for a belief about the past: a belief about what you ate when you had breakfast. And when you clearly “see” or “intuit” that the proposition “If Jack had more than four cups of coffee, then Jack had more than three cups of coffee” is true, then you have evidence for believing that proposition. In this view, evidence consists of perceptual, introspective, memorial, and intuitional experiences, and to possess evidence is to have an experience of that kind. So according to this evidentialism, what makes you justified in believing that p is your having an experience that represents p as being true.
Many reliabilists, too, would say that the experiences mentioned in the previous paragraph matter. However, they would deny that justification is solely a matter of having suitable experiences. Rather, they hold that a belief is justified if, and only if, it results from cognitive origin that is reliable: an origin that tends to produce true beliefs and therefore properly probabilifies the belief. Reliabilists, then, would agree that the beliefs mentioned in the previous paragraph are justified. But according to a standard form of reliabilism, what makes them justified is not the possession of evidence, but the fact that the types of processes in which they originate — perception, introspection, memory, and rational intuition — are reliable.
retirado de: http://plato.stanford.edu/entries/epistemology/#JTB