Using Logic and evidence to Persuade 17.2 379
shift in the wind increase the accuracy of your conclusion that it will rain.
If the non-home-team-T-shirt-wearing students also didn’t attend the pre-
game pep rally or many didn’t show up for campus sports events, those
would be additional signs of student apathy.DeDuCtIve reasonIng According to a centuries-old perspective, rea-
soning from a general statement or principle to reach a specific conclusion is
called deductive reasoning. This is just the opposite of inductive reasoning.
Contemporary logic specialists add that when the conclusion is certain rather
than probable, you are reasoning deductively. The certainty of your conclusion
is based on the validity or truth in the general statement that forms the basis of
your argument.
Deductive reasoning can be structured in the form of a syllogism. A
syllogism is a way of organizing an argument into three elements: a major
premise, a minor premise, and a conclusion:
• Major premise. To reach a conclusion deductively, you start with a gen-
eral statement that serves as the major premise. In a speech attempting to
convince your audience that the communication professor teaching your
public-speaking class is a top-notch teacher, you might use a deductive rea-
soning process. Your major premise is “All communication professors have
excellent teaching skills.”
• Minor premise. the minor premise is a more specific statement about an
example that is linked to the major premise. The minor premise in the argument
you are advancing is “John Smith, our teacher, is a communication professor.”
• Conclusion. The conclusion is based on the major premise and the more spe-
cific minor premise. The conclusion to our syllogism is “John Smith has ex-
cellent teaching skills.”
In reasoning deductively, you need to ensure that both the major premise
and the minor premise are true and can be supported with evidence. The per-
suasive power of deductive reasoning derives from the fact that the conclusion
cannot be questioned if the premises are accepted as true.
Here’s another example you might hear in a speech: Ann was trying to con-
vince the city council to refuse a building permit to Mega-Low-Mart, a large
chain discount store that wants to move into her town. She believed that the
new store would threaten her downtown clothing boutique. Here’s the deduc-
tive structure of the argument she advanced:
Major premise: Every time a large discount store moves into a
small community, the merchants in the downtown
area lose business and the town loses tax revenue
from downtown merchants.
Minor premise: Mega-Low-Mart is a large discount chain that
wants to build a store in our town.