More scholastic material
The modern way of validating a syllogism is with the use of Venn Diagrams, and there are some good web sites that show how to use them, but in the spirit of the middle ages, I'll present the traditional method.
The traditional syllogism has three statements and three terms (Subject, Middle, and Predicate).
The conclusion is of the form Subject / operator / Predicate
Premise 1 relates the middle and predicate terms,
Premise 2 relates the middle and subject,
the conclusion relates subject and predicate.
There are four
Figures:
1. Middle / Predicate
Subject / Middle
Subject / Predicate
2. Predicate / Middle
Subject / Middle
Subject / Predicate
3. Middle / Predicate
Middle / Subject
Subject / Predicate
4. Predicate / Middle
Middle / Subject
Subject / Predicate
We have a concept of "distribution". The members of a class are distributed according to the type of proposition, as follows:
"All" distributes the subject of a proposition.
"None" distributes the subject and predicate.
"Some" distributes neither subject nor predicate.
"Some - not" distributes the predicate (well, not in the sense of the other three, but you just have to accept this)
Distribution, with the exception of type O, thus says something about all members of a class. There are problems with this bald characterization, but you just have to trust me.
Now, four rules for validity (assuming non-empty classes):
Middle term distributed at least once
If a term is distributed in the conclusion, it must be distributed in a premise
At least one non-negative (A or I) premise
If the conclusion is negative (E or O) there must be a negative premise
Example 1 - :
All A are B
All C are A
All C are B
Known as
Barbara. A is the Middle term, B the predicate and C the subject. All the valid figures have traditional names, the vowels of which correspond to the operators in the statements.
I never showed why this was valid did I?
A, the middle term, is distributed in premise 1. C is distributed in the conclusion, and in premise 2. Affirmative premises and conclusion. Valid.
This is of course an instance of the well-known
All men are mortal
All Greeks are men
All Greeks are mortal (or is that "all greeks are Socrates" - I forget)