“That,” said Mr. Winwood, “is a very plausible argument. But, you observe, sir, that it contains an undistributed middle term.”

Thorndyke smiled genially. I think he forgave Winwood everything for that remark.

“You are quite right, sir,” he said. “It does. And, for that reason, the demonstration is not absolute. But we must not forget, what logicians seem occasionally to overlook: that the ‘undistributed middle,’ while it interferes with absolute proof, may be quite consistent with a degree of probability that approaches very near to certainty. Both the Bertillon system and the English fingerprint system involve a process of reasoning in which the middle term is undistributed. But the great probabilities are accepted in practice as equivalent to certainties.”

Mr. Winwood grunted a grudging assent, and Thorndyke resumed: