Cognitive Psychology: Connecting Mind, Research and Everyday Experience, 3rd Edition

362 • CHAPTER 13 Reasoning and Decision Making

To understand why the conclusion does not logically

follow from the two premises, consider ● Figure 13.1. All

of the students are tired (Premise 1) and are sitting in the

student section of the stadium. Some tired people, who

are sitting across the fi eld from the student section, are

irritable (Premise 2). The fact that the tired and irritable

people are sitting across the fi eld from the students is con-

sistent with the second premise because this premise just

says some tired people are irritable, without mentioning

students. Thus, just because the students are tired, and

some tired people are irritable, the conclusion that some

of the students are irritable does not follow. Because this

conclusion does not logically follow from the premises,

this syllogism is not valid.

The procedures for determining validity or lack of

validity are complicated, and are more appropriately cov-

ered in a course in logic. The main message to take away

from our discussion is that “good reasoning” and “truth”

are not the same thing. This can have important implica-

tions for examples of reasoning that you might encounter.

Consider, for example, the following statement:

Listen to me. I know for a fact that all of the members of

Congress from New York are against that new tax law. And

I also know that some members of Congress who are against

that tax law are taking money from special interest groups.

What this means, as far as I can tell, is that some of the mem-

bers of Congress from New York are taking money from spe-

cial interest groups.

What is wrong with this argument? It happens to have

exactly the same form as Syllogism 3, and as with Syllogism 3, it doesn’t logically follow

that just because all of the members of Congress from New York are against the new tax

law (or all students are tired), and some members of Congress who are against the new

tax law are taking money from special interest groups (or some people who are tired

are irritable), that some members of Congress from New York are taking money from

special interest groups (or some students are irritable). Thus, even though syllogisms

may seem “academic,” people often use syllogisms to “prove” their point, often without

realizing that their reasoning might be invalid. It is therefore important to realize that

even conclusions that might sound true are not necessarily the result of good reasoning.

We have been discussing categorical syllogisms, in which the statements begin with

all, no, or some. Another type of syllogism, more commonly encountered in everyday

experience, is the conditional syllogism.

CONDITIONAL SYLLOGISMS

Conditional syllogisms have two premises and a conclusion, like the ones we have been

discussing, but the fi rst premise has the form “If... then... .” This kind of deductive rea-

soning is common in everyday life. For example, let’s say that you lent your friend Steve

$20, but he has never paid you back. Knowing Steve, you might say to yourself that

you knew this would happen. Stated in the form of a syllogism, your reasoning might

look like this: If I lend Steve $20, then I won’t get it back. I lent Steve $20. Therefore, I

won’t get my $20 back.

The four major types of conditional syllogisms are listed in Table 13.1. They are

presented in abstract form (using p and q) and also in the form of a concrete “everyday”

example. For conditional syllogisms, the notations p and q are typically used instead of

the A and B used in categorical syllogisms. The symbol p, the fi rst or “if” term, is called

the antecedent, and q, the second or “then” term, is called the consequent.

T

T

T

T

T

T T

T

T

T T

T T T

T

Student

section

Premise 1:

All students are tired.

Tired student

Premise 2:

Some tired people are irritable.

Person

Tired person

Irritable tired person

Conclusion:

Some of the students are irritable (but not in

this scene, so the syllogism is not valid).

Rest of

the people

T

T

T

T

T

T

T

T

T T

T

T T

T

T

TT T T

T

T

T T

T

T

T

T

T

T

T

T

T

●FIGURE 13.1 When we compare the places where people and

students (who are also people!) are seated in the stadium, we can

see that this seating arrangement is consistent with the fi rst two

premises of Syllogism 3. Note that in this example none of the

students is irritable. Therefore the syllogism is not valid.

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