But in any other sphere of life, a belief is a check that everyone insists upon cashing this side of the grave: the engineer says the bridge will hold; the doctor says the infection is resistant to penicillin—these people have defeasible reasons for their claims about the way the world is. The mullah, the priest, and the rabbi do not. Nothing could change about this world, or about the world of their experience, that would demonstrate the falsity of many of their core beliefs. This proves that these beliefs are not born of any examination of the world, or of the world of their experience. (They are, in Karl Popper's sense, "unfalsifiable.")