Epistemology is a branch of philosophy concerned with the nature of knowledge, how one obtains it, and what constitutes truth. More generally epistemology can be seen as the study of how we know. Epistemology has also been defined as the examination of the obstacles to knowledge, with its goal being to "remove sources of error, rather than to define the nature of truth".
Epistemology is a major problem in economics. Because, in the social sciences, there is no way to isolate variables as one would in a laboratory, it is difficult to prove economic theories by empirical evidence. Therefore, Austrian economics uses deduction to infer economic principles from more basic, a priori principles. Also, the study of history is fraught with unprovable interpretations; there is no way to determine what would have happened if a variable (e.g. a government policy) had been different because the exact set of circumstances cannot be reproduced. Ludwig von Mises wrote a whole book, Epistemological Problems of Economics, on these matters.