 When two or more independent witnesses testify to the truth of the same proposition, and their stories are relatively unlikely, the probability of the proposition in question should increase significantly. This phenomenon of “confidence boost” is known as corroboration. In case of circumstantial evidence, the analogous phenomenon is called convergence. Cohen (1977) argues that no probabilistic measure of evidential support captures this phenomenon. He examines different probabilistic proposals—Boole’s formula (Boole 1857), Ekelöf’s principle (Ekelöf 1964), a formula due to Lambert and Kruskal (1988)—and finds them inadequate. He argues that the confidence boost that is expected from corroboration is not adequately captured by any of these proposals.