They must either believe George or Martha is lying, or that both are mistaken. But they cannot accept both testimonies as true. To believe both testimonies is to accept the thesis that the brother was 500 miles away and not 500 miles away at the same time. Such contradictory evidence is incoherent. There may be reason to believe Martha over George, but both cannot be believed.
Internal coherence is the most decisive test for a set of basic beliefs.
We need to carefully examine the relationship between world view categories. If statements about the nature of reality contradict assertions made about human nature, truth or values, then the world view under investigation is incoherent. Because metaphysics is the most basic world view category, we find incoherence most often in the relationship between it and the other three categories. When naturalists believe in objective moral standards, their world view is incoherent. It is not possible to derive a statement of value in a valueless universe.