 The epistemological puzzle testimony raises is this: Why is testimony a source of knowledge? An externalist might say that testimony is a source of knowledge if, and because, it comes from a reliable source. But here, even more so than in the case of our faculties, internalists will not find that answer satisfactory. Suppose you hear someone saying “p”. Suppose further that person is in fact utterly reliable with regard to the question of whether p is the case or not. Finally, suppose you have no clue whatever as to that person’s reliability. Wouldn’t it be plausible to conclude that, since that person’s reliability is unknown to you, that person’s saying “p” does not put you in a position to know that p? But if the reliability of a testimonial source is not sufficient for making it a source of knowledge, what else is needed? Thomas Reid suggested that, by our very nature, we accept testimonial sources as reliable and tend to attribute credibility to them unless we encounter special contrary reasons. But that’s merely a statement of the attitude we in fact take toward testimony. What is it that makes that attitude reasonable? It could be argued that, in one’s own personal experiences with testimonial sources, one has accumulated a long track record that can be taken as a sign of reliability. However, when we think of the sheer breadth of the knowledge we derive from testimony, one wonders whether one’s personal experiences constitute an evidence base rich enough to justify the attribution of reliability to the totality of the testimonial sources one tends to trust (see E. Fricker 1994 and M. Fricker 2007 for more on this issue). An alternative to the track record approach would be to declare it a necessary truth that trust in testimonial sources is at least prima facie justified. While this view has been prominently defended, it requires an explanation of what makes such trust necessarily prima facie justified. Such explanations have proven to be  controversial.[60] 