in data to captivate our belief in the truth of the data
themselves, even when they are unproved and unprovable. There is no
end, in short, of the ways in which men habitually go wrong in their
reasoning, tacit or expressed. The greatest boon that any benefactor
could confer on the human race would be to teach men--and especially
women--to quantify their propositions. It sometimes seems as if Swift
were right when he said that Mankind were just as fit for flying as
for thinking.

Now it is quite true that mother-wit and the common experiences of
life do often furnish people with a sort of shrewd and sound judgment
that carries them very creditably through the world. They come to good
conclusions, though perhaps they would give bad reasons for them, if
they were forced to find their reasons. But you cannot count upon
mother-wit in everybody; perhaps not even in a majority. And then as
for the experience of life,--there are a great many questions, and
those of the deepest ultimate importance to mankind, in which the
ordinary experience of life sheds no light, until it has been
interrogated and interpreted by men with trained minds. 'It is far
easier,' as has been said, 'to acquire facts than to judge what they
prove.' What is done in our systems of training to teach people how to
judge what facts prove? There is Mathematics, no doubt; anybody who
has done even no more than the first book of Euclid's geometry, ought
to have got into his head the notion of a demonstration, of the
rigorously close connection between a conclusion and its premisses, of
the necessity of being able to show how each link in the chain comes
to be where it is, and that it has a right to be there. This, however,
is a long way from the facts of real life, and a man might well be a
great geometer, and still be a thoroughly bad reasoner in practical
questions.

Again, in other of your classes, in Chemistry, in Astronomy, in
Natural History, besides acquiring groups of facts, the student has a
glimpse of the method by which they were discovered, of the type of
inference to which the discovery conforms, so that the discovery of a
new comet, the detection of a new species, the invention of a new
chemical compound, each becomes a lesson of the most beautiful and
impressive kind in the art of reasoning. And it would be superfluous
and impertinent for me here to point out how valuable such lessons are
in the way of mental discipline, apart from the fruit t