In using an argument from sign care must be taken not to confuse the
relation of cause and effect with that of contiguity in time or place. Do
not allege that which happened at the same time or near the same place as
a cause. If you do use an attendant circumstance, be sure that it adds
something to the probability.
+191. Argument from Example.+--It has been pointed out in the study of
inductive reasoning (Section 176) that a single example may suffice to
establish a general notion of a class. In dealing with objects of the
physical world, if essential and invariable qualities of the object are
considered, they may be asserted to be qualities of each member of the
class, and such an argument from an individual to all the members of the
class is convincing. They thus rank with arguments from sign as effective
in proving the certainty of a proposition.
In dealing with human actions, on the other hand, examples are seldom
proofs of fact. We cannot say that all men will act in a certain way under
given circumstances because one man has so acted. Nevertheless, arguments
by examples are frequently used and are especially powerful when we wish
not only to convince a man, but also to persuade him to action. This
persuasion to action must be based on conviction, and in such a case the
argument from sign that convinces the man of the truth of a proposition
should precede the example that urges him to action. After convincing a
friend that there are advantages to be derived from joining a society, we
may persuade him to join by naming those who have joined.
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