Deductive Reasoning
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A deductive argument begins with premises (reasoning or support for the conclusion) and ends with a
conclusion.
A deductive argument GUARANTEES that is the premises are true, the conclusion is true.
When you argue deductively, you should be able to say “Therefore, ___” with certainty.
The most widely used example of a
deductive argument is…
All humans are mortal. Socrates is
human.
Therefore, Socrates is mortal.
Another
Example:
The members of the Williams family are Susan, Nathan and Alexander.
Susan wears glasses. Nathan wears glasses. Alexander wears glasses.
Therefore, all members of the Williams family wear glasses.
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You can disprove/disagree with a deductive argument in two ways:
o #1. Attack the logic. The logic of a deductive argument relies on the stated premises leading to
the stated conclusion. If that doesn’t happen, the argument is invalid.
o #2. Assert that one (or more) of the premises is (are) untrue. Just because a deductive
argument is logically sound doesn’t make it true. For example…
 All of Mrs. Brown’s students like the band Train. Joe Joe is Mrs. Brown’s student.
Consequently, Joe Joe likes the band Train.
**The major premise that all of my students like the band Train is false (I checked).
Although the argument is logical because the premises lead to the conclusion, you are
disagreeing with the truthfulness of one of those premises.
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When the deductive argument starts with a major premise (SOME/ALL/NO __ are/is ___, like “All men
are mortal”) and moves to a minor premise (a specific premise like “Socrates is a man”), it is called a
syllogism.
o Sample syllogism: All of Mrs. Brown’s students have brown hair. Sally is Mrs. Brown’s student.
Therefore, Sally has brown hair.