Full Idea
It follows from the probability calculus that every tautology has probability 1; it then follows in Bayesian epistemology that we are justified in believing every tautology.
Gist of Idea
Since every tautology has a probability of 1, should we believe all tautologies?
Source
J Pollock / J Cruz (Contemporary theories of Knowledge (2nd) [1999], §4.3.1.5)
Book Reference
Pollock,J.L./Cruz,J: 'Contemporary Theories of Knowledge (2nd)' [Rowman and Littlefield 1999], p.105
A Reaction
If I say 'a bachelor is a small ant' you wouldn't believe it, but if I said 'I define a bachelor as a small ant' you would have to believe it. 'Bachelors are unmarried' men is a description of English usage, so is not really a simple tautology.