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.