  Peirce’s accounts of his third type of deviance from perfect certitude, namely plausibility, are much sketchier than his accounts of probability and verisimilitude. Unlike the other two forms of uncertainty, which can be spelled out mathematically with great precision, plausibility seems to be capable of only a qualitative account, even though plausibility does seem to comes in greater and lesser degree.  The question of the plausibility of a claim arises, apparently, only in contexts in which one is seeking to adduce an explanatory hypothesis for some actual fact that is surprising. The key point is that the hypothesis must be plausible in order to taken seriously.  If we were, for example, to come upon a lump of ice in the middle of a desert, we might plausibly say that perhaps someone put it there, or perhaps a freak storm had left a great hailstone. But we would not plausibly say that it had been thrown off a flying saucer that previously had swooped through. It should be obvious that the notion of plausibility is a difficult one, which strongly invites further analysis but which is not easy to analyze in technical detail.