This week the concept I found the most helpful is the section on
hypothetical syllogisms. Hypothetical syllogisms are a type of deductive argument
that contain an “if…then…” reasoning (conditional statement) as one of two
premises. There are three forms of hypothetical syllogisms: (1) modus ponens, (2)
modus tollens and (3) chain arguments.
(1) In the modus ponens pattern, the first premise is a conditional
statement. The second premise is a sentence that states the “if” part of the
first statement is true. The conclusion follows usually with the word, “therefore.”
In this argument, if the premises are true, then the conclusion must be true
unless the second premise is the “then” statement which may make the argument
invalid.
Example:
If it rains, then
this street will be flooded.
It rained.
Therefore, the
street is flooded.
(2) In the modus tollens pattern, the second premise contradicts the “then”
statement in the first premise. The modus tollens is also an effective way to
argue because again if the premises are true, no matter what they are, then the
conclusion must be true.
Example:
If
there is not enough sunlight, then the plant will die.
The
plant did not die.
Therefore,
there was enough sunlight.
(3) Chain arguments consist of three “if then” statements, two premises
and a conclusion. If a chain argument specifically follows the correct pattern,
then it is a valid argument. You can have more than three conditional
propositions, but it’s no longer a syllogism although it’s still a deductive argument.
Example:
If
the road is blocked, then the cars cannot go through that road.
If the cars cannot
go through that road, they are to take another one.
Therefore, if the
road is blocked, the cars have to take another road.
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