We have emphasized that the form of a logical argument is more important than its content....

We have emphasized that the form of a logical argument is more important than its content. Recall in Example that you saw an argument that sounds reasonable, yet has an invalid form. refer to the tables on that show forms of some standard valid and invalid arguments. In each exercise, explain your thinking. Try to make your examples interesting by referring to topics currently in the news.

Some Invalid Arguments

Determine whether the following arguments are valid:

a) If I have the new calling plan, then I can text my friends for free.

I can text my friends for free.

Therefore, I have the new calling plan.

b) If the moon is made of green cheese, then Justin Bieber has sold more than 1,000 digital

albums.

Justin Bieber has sold more than 1,000 digital albums.

∴ The moon is made of green cheese.

Solution:

a) If we let c represent “I have a new calling plan” and let f represent “I can text my friends for free,” then we can write the argument symbolically as

c → f

f

∴ c

We will make a truth table, as we did in Example 1, with columns for both premises and the conclusion.

Line 3 in the table shows that it is possible for both premises to be true, but the conclusion is false. This means that the argument is invalid.

b) This argument has exactly the same form as the argument in part a), and therefore is also invalid.

Write an argument that sounds unreasonable but has a valid form.