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.

"The thumb-mark is that of Reuben Hornby, a fact that establishes a
_prima facie_ probability that he stole the diamonds. But there is no
evidence that he had access to them, and if he had not, he could not
have made the thumb-mark in the manner and at the time stated.

"But John Hornby may have had access to the previously-made thumb-mark
of Reuben, and may possibly have obtained it; in which case he is almost
certainly the thief.