 Another approach that sees rationality constraints between the two orders as based on taking evidence of one’s expected reliability into account derives the constraints top-down from general, widely held, subjective Bayesian assumptions about evidential support, and explicit representation of second-order reliability claims in higher-order objective probability (Roush 2009). Like approach 2 above it uses subjective conditional probability to express the match that is required between the two orders, but it avoids the consequence we saw in most of those approaches—and in the categorical approach and the other calibration approaches just discussed—that a state of self-doubt combined with knowledge of one’s beliefs is incoherent. Unlike the first two calibration approaches it gives an explanation why calibration is part of rationality; it does this by deriving the constraint from another widely accepted assumption, the Principal Principle.