"Some laborites are
democrats, because All blue collar workers are laborites, and some blue collar workers are
not democrats.
A. When we set up this argument in a standard
form and order syllogism, we obtain ...
Some [blue-collar workers] are
not [democrats]. All [blue-collar workers] are [laborites].
Some [laborites] are [democrats].
1. The Venn diagram shows this argument to be
invalid.
2. The mnemonic mechanism of the syllogism
suggests why this argument is invalid. We can't make the affirmative link between S
and P.
B. Intuitively, most of us think that if a
conclusion is negative, then one premiss must be negative as well, and if the conclusion
is affirmative, neither premiss could be negative in a valid argument. This intuition is
correct.
II. The second quality rule is if either
of the premisses of a valid standard form syllogism is negative, then the conclusion must
also be negative.
A. Reason: If an affirmative conclusion is
entailed, then both premisses must be statements of class inclusion. Since class inclusion
is only obtained by affirmative statements, if the conclusion has one class is partly or
wholly contained in the other, then the premisses must assert that the middle class is
contained by the minor class and contained in the major class.
B. The Fallacy of
Drawing an Affirmative Conclusion from a Negative Premiss is the resultant
fallacy, if the rule does not hold.
C. Note: the syllogism does not have to
be in standard form for use to be able to spot this fallacy. All that we need to see is
the mood. Test your understanding by trying the following problems.
