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DEDUCTION: Part 3 - Page 1-b

Figures and Moods of
The Categorical Syllogism

by Jonathan Dolhenty, Ph.D.

 

Rules for the Syllogistic Figures

The First Figure

M P

S M

S P

The middle term (M) is the subject of the major premise and the predicate of the minor premise.

The rule for the First Figure is: The minor premise must be affirmative and the major premise must be universal.

Explanation of the Rule

First Part: The minor premise must be affirmative.

If the minor premise is negative, the conclusion must then be negative (according to General Rule Number 7) and the major term (P) in the conclusion would be universal. If, however, the minor premise is negative, then the major premise would have to be affirmative.

Both premises are not allowed to be negative (according to General Rule Number 6). Thus the major term (P) would be particular in the major premise, since it is the predicate of an affirmative sentence.

In that case, the major term (P) would be particular in the premise and universal in the conclusion. This violates General Rule Number 2. In order to avoid an illicit major, the minor premise must be affirmative.

Second Part: The major premise must be universal.

If the major premise is particular, the middle term (M) would be a particular term, since it is the subject of this proposition. The minor premise in this figure, however, must be affirmative, in order to be in accord with the first part of this rule.

The middle term (M), because it is the predicate of the minor premise, will be particular here also. If, therefore, the major premise is particular, the middle term (M) would be particular in both premises. But this would entail an undistributed middle and violate General Rule Number 4.

The only means by which to get a distributed middle is to make the major premise universal. This shows, then, that the second part of the rule is correct: The major premise must be universal.

The Valid Moods of the First Figure

When we consider the moods which are valid for the syllogisms of the First Figure, we find that four of the eight possible legitimate combinations are valid and four are invalid. Here are the diagrams:

Moods of the First Figure
A
uM
+
pP
A
uS
+
pM

uS
+
pP

Valid
A
uM
+
pP
E
uS
-
uM

uS
-
uP

Illicit Major
A
uM
+
pP
I
pS
+
pM

pS
+
pP

Valid
A
uM
+
pP
O
pS
-
uM

pS
-
uP

Illicit Major
E
uM
+
uP
A
uS
+
pM

uS
-
uP

Valid
E
uM
-
uP
I
pS
+
pM

pS
-
uP

Valid
I
pM
+
pP
A
uS
+
pM

uS
+
pP

Undist.Middle
O
pM
-
uP
A
uS
+
pM

uS
-
uP

Undist.Middle

Clearly, there are only four valid moods for the First Figure: A A, A I, E A, and E I. Each one follows the rule that the minor premise must be affirmative and the major premise must be universal.

The A E and A O moods are invalid because of an illicit major brought into the conclusion. They violate the first part of the rule: The minor premise must be affirmative.

The I A and O A moods are invalid because an undistributed middle is the result of the arrangement and this violates the second part of the rule: The major premise must be universal.

Here are some arguments which illustrate the valid moods for the First Figure:

A

Every human being should have equal rights.

A

Women are human beings.

Therefore, Women should have equal rights.

A

All politicians are great talkers.

I

Some lawyers are politicians.

Therefore, Some lawyers are great talkers.

E

No mammals are cold-blooded.

A

All dogs are mammals.

Therefore, No dogs are cold-blooded.

E

No misers are generous.

I

Some wealthy people are misers.

Therefore, some wealthy people are not generous.

 

To Page 1-c of Figures and Moods of the Syllogism


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