 The concept of the a priori is fundamentally a concept of independence from experience. The a priori stands in contrast to the a posteriori or empirical. Knowledge is empirical just in case it is derived from experience. What experience includes is contested. Experience uncontroversially includes sense perception. It arguably also includes introspection and the testimony of others. Much of what we know is empirical: we know by sense perception or introspection or the testimony of others, etc. A priori knowledge—if there is any—would be independent of sense perception, introspection, etc. (see entry on  rationalism vs. empiricism).  A majority of epistemologists could probably agree that justification is necessary for knowledge if one interprets justification broadly (e.g., so that a belief might be justified in virtue of being produced by a reliable cognitive mechanism, or by an exercise of intellectual virtue, or by having supporting evidence from good reasons). For the purposes of this entry, we will understand justification broadly and assume that in order to have a priori knowledge of a proposition one’s belief in that proposition must be justified (see entries on  the analysis of knowledge;  foundationalist theories of epistemic justification;  coherentist theories of epistemic justification;  internalist vs. externalist conceptions of epistemic justification).