Converstion
Convertend
Converse
A: All S is P
I: Some P is S
(limit)
E: No S is P
E: No P is
S
I: Some S is P
I: Some P is S
O: Some S is ~P
not valid
Obversion
Obvertend
Obverse
A: All S is P
E: No S is ~P
E: No S is P
A: All S is ~P
I: Some S is P
O: Some S is not ~P
O: Some S is ~P
E: Some S is ~P
Contraposition
Premise
Contrapositive
A: All S is P
A: All ~P is ~S
E: No S is P
O:Some ~P is not
~S
I: Some S is P
not valid
O: Some S is ~P
O: Some ~P is not
~S
Contradictories: cannot both be true and
cannot both be false
Contraries: Cannot both be true
Sub contraries: Cannot both be false,
although both maybe true
Syllogism
S: Subject of the conclusion
P: predicate of the conclusion
M: Middle term
First Figure
Second Figure
M is P
P is M
S is M
S is M
So S is P
So S is P
Third Figure
Fourth Figure
M is P
P is M
M is S
M is S
So S is P
So S is P
Syllogistic Rules and Fallacies
Rule
Associated
Fallacy
1. Avoid four terms
Four terms
2. Distriubted the
middle term in at
least one premise
undistributed
middle
3. Any term
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 Fall '09
 Doyle
 Contradictories, Amphiboly Loose, ffi rmative conclusion, ~P Obversion Obvertend, ~P Cont raposition

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