Syllogism is a way or arguing in which
two statements are used to prove that a third statement is true. For example,
All human must die; I am a human; Therefore, I must die. (Oxford Advanced
Learner’s Dictionary). Theory of syllogism was
developed by the Greek philosopher Aristotle.
Conditional
syllogisms
Conditional syllogisms are better
known as hypothetical syllogisms, because the arguments used here are not
always valid. The basic of this syllogism type is: if A is true then B is true
as well. An example will follow to elucidate the former.
Major premise: If Johnny is eating sweets every day, he is
placing
himself at risk for
diabetes.
Minor premise: Johnny does not eat sweats everyday
Conclusion: Therefore Johnny is not placing himself
at risk for
diabetes.
Categorical
syllogisms
The third and most commonly used type
of syllogisms are the categorical syllogisms. The basic for this syllogism type
is: if A is a part of C, then B is a part of C (A and B are members of C). An
example of this syllogism type will clarify the above:
Major premise: All men are mortal.
Minor premise: Socrates is a man.
Conclusion: Socrates is mortal.
RULES OF SYLLOGISM
Rule 1: There must be three terms and only
three – the major term, theminor term, and the middle term. If there are only
two terms therelationship between these two cannot be established. And if there
were more than three terms this would violate the structure of the
categorical syllogism.
Rule 2: Each term must occur twice in the
syllogism: the major must occur in the conclusion and in one premise, the
minor in the conclusion and in one premise; the middle in both premise but
not in the conclusion. There must therefore be a total of three
propositions in the syllogism.
Rule 3: The middle term must be
distributed at least once. If the middleterm is particular in both premises it
might stand for a different portion of its extension in each occurrence
and thus be equivalent to two terms.
Rule 4: The major and minor terms may not
be universal in the conclusion unless they are universal in the premises.
If a term is distributed in the conclusion then it must be distributed
first in the premise.
Rule 5: If both premises are affirmative,
the conclusion must beaffirmative. The reason for this rule is that affirmative
premises either unite the minor or major terms, or else do not bring them
into relationship with each other at all.
Rule 6: If one premise is affirmative and
the other negative, the conclusion must be negative.
Rule 7: If both premises are negative – and
not equivalently affirmative – there can be no conclusion.
Rule 8: If both premises are particular
there can be no conclusion.
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