 On this theory, the evidence for one perceptual belief consists in part of other perceptual beliefs. For instance, the belief that a purported apple is a real apple may find evidential support from perceptual beliefs about what happens when it is exposed to air. It may happen that belief that one object exists supports believe that another does and it, in turn, supports believe that the first exists. For instance, the claim that there is a fire might be based on the observation that it was caused by steel striking flint; but then the external existence of the steel and flint might be confirmed by observing that their concussion caused fire. Shepherd notes that someone might think this is arguing in a circle. To put it differently, it might seem to us that every perceptual belief receives its evidential value from nothing but other perceptual beliefs.  Shepherd sees it differently. Other perceptual beliefs are relevant only because we understand that if the actual cause of, say, a purported fire is the apparent cause, then it is necessary that the unknown cause at work in the purported case has all the qualities in the definition of fire. So CP and CPI are foundational in Shepherd’s theory of probable belief. Still, we just saw that the probability that the apparent cause of a given effect is the actual cause depends on other perceptual beliefs (ERCE: 124–5). 