"Datum 1: He knows a merchantman was sunk," pipes up Turing, ticking off points on his fingers. "Datum 2: He knows a Royal Navy submarine was on the scene a few hours later, and was also sunk. Datum 3: He knows two of our men were pulled out of the water, and that they are probably in the intelligence business, which is a rather broad categorization as far as I am concerned. But he cannot necessarily draw any inferences, based upon these extremely terse messages, about which vessel—the merchantman or the submarine—our two men came from."