gro-lethargy as it is sometimes called; and here we are in a
difficulty. We have not enough knowledge. Neither of us, I take it, has
ever seen a case, and the extant descriptions are inadequate. From what
I know of the disease, its symptoms agree with those in your case in
respect of the alleged moroseness and in the gradually increasing
periods of lethargy alternating with periods of apparent recovery. On
the other hand, the disease is said to be confined to negroes; but that
probably means only that negroes alone have hitherto been exposed to the
conditions that produce it. A more important fact is that, as far as I
know, extreme contraction of the pupils is not a symptom of sleeping
sickness. To sum up, the probabilities are against sleeping sickness,
but with our insufficient knowledge, we cannot definitely exclude it."

"You think that it may really be sleeping sickness?"

"No; personally I do not entertain that theory for a moment. But I am
considering the evidence apart from our opinions on the subject. We have
to accept it as a conceivable hypothesis that it may be sleeping
sickness because we cannot positively prove that it is not. That is all.
But when we come to the hypothesis of morphine poisoning, the case is
different. The symptoms agree with those of morphine poisoning in every
respect. There is no exception or disagreement whatever. The common
sense of the matter is therefore that we adopt morphine poisoning as our
working diagnosis; which is what you seem to have done."

"Yes. For purposes of treatment."

"Exactly. For medical purposes you adopted the more probable view and
dismissed the less probable. That was the reasonable thing to do. But
for legal purposes you must entertain both possibilities; for the
hypothesis of poisoning involves serious legal issues, whereas the
hypothesis of disease involves no legal issues at all."

"That doesn't sound very helpful," I remarked.

"It indicates the necessity for caution," he retorted.

"Yes, I see that. But what is your own opinion of the case?"

"Well," he said, "let us consider the facts in order. Here is a man who,
we assume, is under the influence of a poisonous dose of morphine. The
question is, did he take that dose himself or was it administered to him
by some other person? If he took it himself, with what object did he
take it? The history that was given to you seems completely to exclude
the idea of suicide. But the patient's condition seems