The Language of Argument

1 3 9

C o n d i t i o n a l s

The truth table showing the invalidity of this argument form looks like this:

Premise Premise Conclusion

p q p ⊃ q ~p ~q

T T T F F

T F F F T

F T T T F Invalid

F F T T T OK

A second standard fallacy is called affirming the consequent. It looks like this:

p ⊃ q

q

∴ p

Use truth tables to show that this argument form is invalid.

Exercise XXI

In his radio address to the nation on April 17, 1982, President Ronald Reagan

argued that the United States should not accept a treaty with the Soviet Union

that would mutually freeze nuclear weapons at current levels, because he

believed that the United States had fallen behind. Here is a central part of his

argument:

It would be wonderful if we could restore the balance of power with the

Soviet Union without increasing our military power. And, ideally, it would

be a long step towards assuring peace if we could have significant and ver-

ifiable reductions of arms on both sides. But let’s not fool ourselves. The

Soviet Union will not come to any conference table bearing gifts. Soviet ne-

gotiators will not make unilateral concessions. To achieve parity, we must

make it plain that we have the will to achieve parity by our own effort.

Put Reagan’s central argument into standard form. Then symbolize it and its

form. Does his argument commit any fallacy? If so, identify it.

Exercise XXII

The relations among these last four argument forms can be seen in this

diagram:

Antecedent   Consequent

Affirming Affirming the Antecedent =     Affirming the Consequent

Modus Ponens (valid)     (invalid)

Denying Denying the Antecedent   Denying the Consequent =

(invalid)    Modus Tollens (valid)

97364_ch06_ptg01_111-150.indd 139 15/11/13 10:15 AM

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