The Language of Argument

8 8

cH A Pt eR 5 ■ D e e p A n a l y s i s

Although this argument has two premises, it does not give two separate reasons

for its conclusion. Neither premise by itself, without the other, is enough to give

us any reason to believe the conclusion: “My keys are either at my home or at

my office” alone is not enough to support “My keys are at my home,” and “My

keys are not at my office” alone is also not enough to support “My keys are at

my home.” The premises work only when they work together. Thus, it would be

misleading to diagram this argument in the same way as Kyl’s argument.

Instead, we need to indicate that the premises work together. Here’s a

simple way:

(1) + (2)

(3)

The symbol “+” with a single arrow indicates that the two premises work to-

gether to provide a single reason for the conclusion. The line under the premises

that are joined together makes it clear that those are the premises that lead to the

conclusion at the end of the arrow. If three or more premises provided a single

reason, then we could simply add to the list—(1) + (2) + (3), and so on—then

draw a line under the premises to show which ones work together. Because these

premises work jointly rather than separately, we can call this structure joint.

The argument that we are diagramming included one part that we have

not incorporated yet:

They can’t be at the office, because I looked for them there.

The standard form is this:

(2*) I looked for my keys at my office.

∴(2) My keys can’t be at my office. (from 2*)

By itself, this argument has this diagram:

(2*)

(2)

Since the conclusion of this background argument is a premise in the other

part of the argument, we can put the diagrams together like this:

(2*)

(1) + (2)

(3)

The fact that the arrow goes from (2*) to (2) but not to (1) indicates that this

background argument supports premise (2), but not the other premise. In

cases like this, you need to be careful where you draw your arrows.

Argument structures can get very complex, but we can diagram most argu-

ments by connecting the simple forms that we illustrated. Begin by identifying

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