In a book entitled Principles of the Algebra of Logic, which deals with the Sciences of Necessity and Probability as one whole, I have considered as an example the question in probabilities discussed in vol. xxi (p. 204) of this Journal, namely—“ A says “ that B says that a certain event took place : required the pro- “ bability that the event did take place, p1 and p2 being A’s and “ B’s respective probabilities of speaking the truth.”