ases where it breaks down. Perhaps somebody makes
the general statement whose authority you do not accept; perhaps he
says it in an assertive way that makes you want to take him down {473}
a peg. Perhaps you are in the heat of an argument with him, so that
every assertion he may make is a challenge. You search your memory for
instances belonging under the doubted general statement, in the hope
of finding one where the general statement leads to a result that is
contrary to fact. "You say that all politicians are grafters. Theodore
Roosevelt was a politician, therefore, according to you, he must have
been a grafter. But he was not a grafter, and you will have to take
back that sweeping assertion."

6. Verification.

This same general type of reasoning, which takes its start with a
general proposition, and explores particular instances in order to see
whether the proposition, when applied to them, gives a result in
accordance with the facts, has much more serious uses; for this is the
method by which a _hypothesis_ is tested in science. A hypothesis is a
general proposition put forward as a guess, subject to verification.
If it is thoroughly verified, it will be accepted as a true statement,
a "law of nature", but at the outset it is only a guess that may turn
out to be either true or false. How shall its truth or falsity be
demonstrated? By deducing its consequences, and testing these out in
the realm of observed fact.

An example from the history of science is afforded by Harvey's
discovery of the circulation of the blood, which was at first only a
hypothesis, and a much-doubted one at that. If the blood is driven by
the heart through the arteries, and returns to the heart by way of the
veins, then the flow of blood in any particular artery must be away
from the heart, and in any particular vein towards the heart. This
deduction was readily verified. Further, there should be little tubes
leading from the smallest arteries over into the smallest veins, and
this discovery also was later verified, when the invention of the
microscope made observation of the capillaries possible. Other
deductions also were verified, {474} and in short all deductions from
the hypothesis were verified, and the circulation of the blood became
an accepted law.

Most hypotheses are not so fortunate as this one; most of them die by
the wayside, since it is much easier to make a guess that shall fit
the few facts we already know than to mak