Caption: Venn diagram illustrating what yours truly calls the A-principle and its generalization by Bayes' theorem which is the root of Bayesian analysis.
The Venn diagram makes the A-principle clear.
The contour lines delimit statistical samples (i.e., subsets of the statistical population).
Since sample A is entirely inside sample B, we have conditional probability P(B|A) = 1 which is the A-principle itself.
Logically equivalently, P(A|[not B]) = 0.