378 17.2 Using PersUasive strategies
drawn an analogy, reasoning from one example to reach a conclusion about
the other. If you try to convince an audience that because laws against using a
cell phone while driving in a school zone have cut down on injuries to children
in Florida and Missouri, those laws should therefore be instituted in Kansas,
you are reasoning by analogy. You would also be reasoning by analogy if you
claimed that because capital punishment reduced crime in Brazil, it should be
used in the United States as well. But as with reasoning by generalization, there
are questions that you should ask to check the validity of your conclusions:
• Do the ways in which two things are alike outweigh the ways in which they are
different? Can you compare the crime statistics of Brazil to those of the
United States and claim to make a valid comparison? Could other factors
besides the cell phone laws in Florida and Missouri account for the lower
automobile accident death rate? Maybe differences in speed limits in school
zones can account for the difference.
• Is the assertion true? Is it really true that capital punishment has deterred
crime in Brazil? You will need to give reasons the comparison you are mak-
ing is valid and evidence that will prove your conclusion true.
reasonIng by sIgn reasoning by sign, another special type of inductive
reasoning, occurs when two things are so closely related that the existence of one
thing means that the other thing will happen. For example, white smoke billow-
ing from the chimney of the Sistine Chapel in Rome’s Vatican Square is a sign
that there is a new pope. A clap of thunder and dark, swirling clouds are signs of
rain. One specific sign, or presence of multiple signs, leads you to a conclusion
that something else has happened or will happen.
One student group observed that many students on campus were wearing
T-shirts promoting other college and university sports teams rather than the
teams on their home campus. They viewed the T-shirts of other teams as a sign
of student apathy, deflated school spirit, and disinterest in campus sports. To fix
the problem, the group wanted to ban students from wearing shirts promoting a
team other than the local campus team.
When you use or hear reasoning by sign, consider these questions:
• Is there a strong, predictive relationship between the sign and the asserted conclu-
sion? If white smoke always precedes the announcement of a new pope,
then there is a strong relationship in which the sign (white smoke) predicts
the conclusion (there is a new pope). Does the shirt someone wears (a sign)
always predict a lack of school spirit?
• Is there another explanation for the relationship between the sign and the asserted
conclusion? Simply because students wear shirts promoting another team,
does that mean students really don’t care about the home team? Perhaps the
students just couldn’t afford to buy new shirts.
• Are there multiple signs? Hearing only thunder may not mean it will rain,
but hearing thunder, seeing lightning and dark clouds, and feeling a sudden