 This second horn is sharpened by White (2006), who formalizes it in probabilistic terms. Let \(A(D)\) be the proposition that there appears to be a door before you, and \(D\) the proposition that there really is a door there. The conjunction \(A(D) \wedge \neg D\) represents the possibility that appearances are misleading in this case. It says there appears to be a door but isn’t really. Using the probability axioms, we can prove that \(p(D\mid A(D)) \leq p(\neg (A(D) \wedge \neg D))\) (see  technical supplement §3).  In other words, the probability that there really is a door given that there appears to be one cannot exceed the initial probability that appearances are not misleading in this case. So it seems that any justification \(A(D)\) lends to belief in \(D\) must be preceded by some justification for believing that appearances are not misleading, i.e., \(\neg (A(D) \wedge \neg D)\). Apparently then, you must know (or have reason to believe) your sources are reliable before you can trust them. (Pryor 2013 elucidates some tacit assumptions in this argument.)