oundation of most
arguments on large questions. If you were arguing in favor of the
purchase of local waterworks, you would present figures showing the
number of houses using the public water supply, the rates paid, the
profits of the company, the exact points at which public control could
work economies. If you were arguing for a rule that no man shall play on
a university team until he has been registered a year at the university,
you would need statistics to show how many men would be affected by the
rule. If you were arguing for a single session at a school instead of
two, you would show exactly how many students in the school live more
than a mile away from the building. In every case where statistics can
be presented in such a way as to make clear that they fairly cover the
ground, they are most valuable evidence. They give the argument the
effect of being founded on a rock. If it be obvious that the statistics
have been freshly gathered, and are not merely casual and second-hand
gleanings, they have still greater effect, for then they have a
secondary force in testifying to the personal knowledge that the witness
has of the subject. We shall see later the danger of the fallacy of
generalizing on too narrow a basis: a generalization based on a good
body of statistics runs no danger of this fallacy.

31. The Opinion of Recognized Authorities. The other chief source
of evidence to establish a fact which consists of a large and complex
state of affairs is the opinion of recognized authorities on the
subject. The strength of such evidence depends on whether the audience
will accept the person you cite as having authority on the matter. Most
of us read some newspaper or periodical in the opinions of which we have
confidence, because they seem to be based on investigation and competent
knowledge. The annual report of the Secretary of the Treasury is
excellent evidence on the state of the national finances. The reports of
presidents of colleges are excellent evidence from authorities on such
questions as the value of the elective system or the effect of raising
the standard of admission. The report of a dean or of a schoolmaster on
the value of organized athletics is effective if the audience knows that
he likes out-of-door sports and takes time to see the games. Evidence
drawn from an authority who is likely to be used by the other side is
doubly effective, since your readers recognize that his competence is
admitted.