"John Blackmore did not reside at New Inn, but at some place within easy reach of it. But Weiss resided at a place within easy reach of New Inn.
"John Blackmore must have had possession and control of the person of Jeffrey. But Weiss had possession and control of the person of Jeffrey.
"Weiss wore spectacles of a certain peculiar and probably unique character. But portions of such spectacles were found in the chambers at New Inn.
"The overwhelming probability, therefore, is that Weiss and the tenant of New Inn were one and the same person; and that that person was John Blackmore."
"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:
"We have now furnished fairly conclusive evidence on three heads: we have proved that the sick man, Graves, was Jeffrey Blackmore; that the tenant of New Inn was John Blackmore; and that the man Weiss was also John Blackmore. We now have to prove that John and Jeffrey were together in the chambers at New Inn on the night of Jeffrey's death.
"We know that two persons, and two persons only, came from Kennington Lane to New Inn. But one of those persons was the tenant of New Inn—that is, John Blackmore. Who was the other? Jeffrey is known by us to have been at Kennington Lane. His body was found on the following morning in the room at New Inn. No third person is known to have come from Kennington Lane; no third person is known to have arrived at New Inn. The inference, by exclusion, is that the second person—the woman—was Jeffrey.