"No," he admitted. "But the kind of syllogism that they do make is this—
"'The crime was committed by the person who made this finger-print.
"'But John Smith is the person who made the finger-print.
"'Therefore the crime was committed by John Smith.'"
"Well, that is a perfectly good syllogism, isn't it?" I asked.
"Perfectly," he replied. "But, you see, it begs the whole question, which is, 'Was the crime committed by the person who made this finger-print?' That is where the corroboration is required."
"That practically leaves the case to be investigated without reference to the finger-print, which thus becomes of no importance."
"Not at all," rejoined Thorndyke; "the finger-print is a most valuable clue as long as its evidential value is not exaggerated. Take our present case, for instance. Without the thumb-print, the robbery might have been committed by anybody; there is no clue whatever. But the existence of the thumb-print narrows the inquiry down to Reuben or some person having access to his finger-prints."
"Yes, I see. Then you consider my theory of John Hornby as the perpetrator of the robbery as quite a tenable one?"
"Quite," replied Thorndyke. "I have entertained it from the first; and the new facts that you have gathered increase its probability. You remember I said that four hypotheses were possible: that the robbery was committed either by Reuben, by Walter, by John Hornby, or by some other person. Now, putting aside the 'some other person' for consideration only if the first three hypotheses fail, we have left, Reuben, Walter, and John. But if we leave the thumb-print out of the question, the probabilities evidently point to John Hornby, since he, admittedly, had access to the diamonds, whereas there is nothing to show that the others had. The thumb-print, however, transfers the suspicion to Reuben; but yet, as your theory makes evident, it does not completely clear John Hornby. As the case stands, the balance of probabilities may be stated thus: John Hornby undoubtedly had access to the diamonds, and therefore might have stolen them. But if the thumb-mark was made after he closed the safe and before he opened it again, some other person must have had access to them, and was probably the thief.