The Problem of A, B, C and D

Extract

If A, B, C and D each speak the truth once in three times (independently), and A affirms that B denies that C declares that D is a liar, what is the probability that D was speaking the truth?

I understand that this problem appeared many years ago in a mock examination paper in the Granta. I heard of it from Dr. A. C. D. Crommelin in 1919, when A, B, G and D (or more strictly C, C’, D and E) were about to start on the eclipse expeditions which verified Einstein’s prediction of the deflection of light. Crommelin in an after-dinner speech hinted that this problem might arise. I made use of it in my book New Pathways in Science, and rashly gave my solution-—the result being a considerable increase in my correspondence. I hope the following explanation will be sufficiently clear to avert a similar sequel to its appearance in the Mathematical Gazette.