he brakeman's mind, bred of habit. But if the
passenger had been an acute reasoner ... [and had] singled out of
all the numerous points involved in a stove's not smoking the one
special point of smoke pouring freely out of the stove-pipe's mouth,
he would probably ... have been immediately reminded of the law that
a fluid passes more rapidly out of a pipe's mouth if another fluid
be at the same time streaming over that mouth.[32]

Here the passenger's certainty that the smoking would stop would have
been much increased if he had, as Professor James suggests, reasoned to
the cause, instead of trusting to the brakeman's generalization from
experience.

In scientific matters search for cause and effect the chief mode of
progress. General Sternberg's article "Yellow Fever and Mosquitoes"
(p. 251) is an admirable account of this advance from probability to
certainty, which comes from demonstrating the necessary sequence which
we call cause and effect. When Major Reed and his associates had shown
that in cases where mosquitoes were kept away there was no yellow fever,
but that in cases where infected mosquitoes were allowed to bite
patients yellow fever followed, they turned the probability that
mosquitoes were the transmitting agent of the fever into a certainty.
Likewise with the glacial theory: it had already in the time of the
elder Professor Agassiz been established that certain regions of
northern Europe and America could be classed together by the occurrence
of certain phenomena--rounded hills, ledges of rock smoothed off and
marked with scratches running more or less north and south, deposits of
clean gravel and sand, boulders of various foreign kinds of rock
scattered over the surface of the country; when he showed that glaciers
in their movements produce all these phenomena, he laid bare the cause
of the phenomena, and so demonstrated with practical certainty the
theory of the former existence of a huge glacial sheet in the northern
hemisphere. Wherever you can show that your case not only belongs to a
recognized class of cases, with recognized characteristics, but also
that in those characteristics there is a necessary sequence of cause and
effect, you have proved your point.

In the example above, of an argument for the establishment of a
municipal gymnasium, if after showing that all the boys and young men
who get into the courts have no normal and healthy way of working off
their natur