his; and I may say that I did not
believe it at the time, but merely adopted it as a proposition that was
worth testing. I accordingly tested it, 'Yes?' or 'No?' with each new
fact; but as each new fact said 'Yes,' and no fact said definitely 'No,'
its probability increased rapidly by a sort of geometrical progression.
The probabilities multiplied into one another. It is a perfectly sound
method, for one knows that if a hypothesis be true, it will lead one,
sooner or later, to a crucial fact by which its truth can be
demonstrated.

"To resume our argument. We have now set up the proposition that John
Blackmore was the tenant of New Inn and that he was personating Jeffrey.
Let us reason from this and see what it leads to.

"If the tenant of New Inn was John, then Jeffrey must be elsewhere,
since his concealment at the inn was clearly impossible. But he could
not have been far away, for he had to be producible at short notice
whenever the death of Mrs. Wilson should make the production of his
body necessary. But if he was producible, his person must have been in
the possession or control of John. He could not have been at large, for
that would have involved the danger of his being seen and recognized. He
could not have been in any institution or place where he would be in
contact with strangers. Then he must be in some sort of confinement. But
it is difficult to keep an adult in confinement in an ordinary house.
Such a proceeding would involve great risk of discovery and the use of
violence which would leave traces on the body, to be observed and
commented on at the inquest. What alternative method could be suggested?

"The most obvious method is that of keeping the prisoner in such a state
of debility as would confine him to his bed. But such debility could be
produced by only starvation, unsuitable food, or chronic poisoning. Of
these alternatives, poisoning is much more exact, more calculable in its
effect and more under control. The probabilities, then, were in favour
of chronic poisoning.

"Having reached this stage, I recalled a singular case which Jervis had
mentioned to me and which seemed to illustrate this method. On our
return home I asked him for further particulars, and he then gave me a
very detailed description of the patient and the circumstances. The
upshot was rather startling. I had looked on his case as merely
illustrative, and wished to study it for the sake of the suggestions
that it might offer.