  We give a proof of Cox's Theorem on the product rule and sum rule for
conditional plausibility without assuming continuity or differentiablity of
plausibility. Instead, we extend the notion of plausibility to apply to
unknowns giving them plausible values. Our proof is enormously simpler than
others that have recently appeared in the literature, yet completely rigorous.
For example, we do not need to investigate the 11 possibilities for conditional
plausibilities as described on page 25 of Jaynes' recent book Probability
Theory.
