convince the public that the prophylactic has reduced the
percentage by eighty per cent instead of increasing it by five,
because the public, left to itself and to the old gentlemen who
are always ready to remember, on every possible subject, that
things used to be much worse than they are now (such old
gentlemen greatly outnumber the laudatores tempori acti), will
assume that the former percentage was about 100. The vogue of the
Pasteur treatment of hydrophobia, for instance, was due to the
assumption by the public that every person bitten by a rabid dog
necessarily got hydrophobia. I myself heard hydrophobia discussed
in my youth by doctors in Dublin before a Pasteur Institute
existed, the subject having been brought forward there by the
scepticism of an eminent surgeon as to whether hydrophobia is
really a specific disease or only ordinary tetanus induced (as
tetanus was then supposed to be induced) by a lacerated wound.
There were no statistics available as to the proportion of dog
bites that ended in hydrophobia; but nobody ever guessed that the
cases could be more than two or three per cent of the bites. On
me, therefore, the results published by the Pasteur Institute
produced no such effect as they did on the ordinary man who
thinks that the bite of a mad dog means certain hydrophobia. It
seemed to me that the proportion of deaths among the cases
treated at the Institute was rather higher, if anything, than
might have been expected had there been no Institute in
existence. But to the public every Pasteur patient who did not
die was miraculously saved from an agonizing death by the
beneficent white magic of that most trusty of all wizards, the
man of science.

Even trained statisticians often fail to appreciate the extent to