Epistemology is a branch of philosophy that studies the nature and basis of knowledge. How do we know things? It also studies the veracity of "truth." How do we know the difference between belief, knowledge, opinion, fact, reality and fantasy? The Greek philosopher, Carneades, believed that knowledge of reality, of what is true or false, is impossible, that nothing can be known with certainty; his philosophy is known as skepticism. It does not reject belief altogether; Carneades felt that our belief about any given matter should be subjected to intense scrutiny and then, using a scale of probability, we should accept or reject the likelihood of its truth or falsehood. But we must make no absolute claims to it. Another Greek skeptic, Cratylus, however, was more radical in his approach and believed that nothing could be known at all, and thus no statements could convey anything true or meaningful. He finally gave up talking altogether.
Most of us are neither moderate nor extreme skeptics; we believe what our teachers told us. Although some of us learned later that perhaps a little skepticism was indeed warranted, we survived with our grasp of reality reasonably intact. We live in a world where facts are meaningful and opinions can be assessed, at least to the degree that we deem them sound or unsound. When it comes to religion, those of us who are raised in traditions often reject such assessments and simply believe what we were taught. For many religious people, skepticism is anathema, the work of the devil. However, our Abrahamic traditions of Judaism, Christianity and Islam have always been concerned with and seriously interested in epistemology, because each of these faiths have profound truth claims that need substantiation or "believability."
Islam, at its advent, developed a sophisticated methodology for the validation of truth claims. One of the greatest achievements of the Islamic scholastic tradition is 'ilm ar-rijaal, the science of narrators. It is the study of reports of events in the life of the Prophet, especially of his sayings and deeds. Its formulators established a rigid set of criteria to validate the truth claims of those who asserted they saw or heard the Prophet do or say such-and-such. Reports were grouped into two categories: ahad, or solitary reports in which one or a few people claimed to have heard or seen something, and mutawatir, or multiply-transmitted reports narrated in numbers large enough to preclude collusive fabrication. The solitary reports must meet many criteria before being accepted as sound statements that nonetheless contain, depending upon the degree to which the criteria were met, a certain probability of error. On the other hand, firmly established multiply-transmitted reports, in numbers that rule out collusion, are taken as uncontestable fact.