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Transcript of Headline 5
Rule: Avoid two negative premises
Rule: Any term distributed in the conclusion must be distributed in the premises
Thou shalt not commit fallacies
Fallacy Of The Undistributed Middle
Fallacy Of Illicit Process
Fallacy Of Exclusive Premises
This fallacy is committed in a categorical syllogism when it becomes invalid because both of its premises are negative. Any negative proposition denies class inclusion; it asserts that some or all of one class is excluded from the whole of the other class. When both premises are exclusive, a link to the conclusion cannot be established and therefore the argument becomes invalid.
Major Premise: No animals are insects.
Minor Premise: No insects are dogs.
Conclusion: Therefore, no dogs are animals
Sunday, March 6, 2016
By Group 4
Fallacy Of Four Terms
Rule: If either premise is negative the conclusion must be negative
Fallacy Of Drawing An Affirmative Conclusion From A Negative Premise
This fallacy occurs when a syllogism has four (or more) terms rather than the requisite three. This form of argument is thus invalid.
Major premise: All canines have tails.
Minor premise: All dogs are canines.
Conclusion: Therefore, all cats are canines.
It occurs most frequently by equivocation: using the same word or phrase but with a different meaning each time, creating a fourth term even though only three distinct words are used.
Major Premise: Noisy children are a real headache.
Minor Premise: Two aspirins will make a headache go away.
Conclusion: Therefore, two aspirins will make noisy children go away.
As the name states, this fallacy is committed when a positive conclusion is drawn even when one or more of the premises given are negative and therefore the argument becomes invalid. If the conclusion is affirmative – that is, if it asserts that one of the two classes, is wholly or partly contained in the other- it can only be inferred from premises that assert the existence of a third class that contains the first and is itself contained in the second. However, class inclusion can be stated only by affirmative propositions. Therefore an affirmative conclusion can follow validly only from two affirmative premises.
Major Premise: No people under the age of 66 are senior citizens.
Minor Premise: No senior citizens are children.
Conclusion: Therefore, all people under the age of 66 are children.
Rule: Avoid four terms
This fallacy is committed when the middle term in a categorical syllogism is not distributed in either the minor or the major premise. A term is “distributed” in a proposition when the middle proposition refers to all members of the class designated by that term. In a valid syllogism the middle term must be distributed in at least one premise.
Major Premise: All students carry backpacks
Minor Premise: My grandfather carries a backpack
Conclusion: Therefore, my grandfather is a student.
To refer to all members of a class means that more is being said about the class when only some of its members are being referred to. Thus, a conclusion of a syllogism cannot distribute a term that was undistributed in the premises. A term that is distributed in the conclusion but is not distributed in the premises entails that the conclusion has gone too beyond its premises and has reached too far.
The conclusion may overreach with respect to either the minor term (subject) or the major term (predicate). The two forms of illicit process are therefore: Illicit process of the major term and illicit process of the minor term. Illicit process of the major term occurs when the major term is distributed in the conclusion, but not in the major premises. Illicit process of the minor term occurs when the minor term is distributed in the conclusion but not in the minor premises.
Example of Illicit process of major term:
Major Premise: All hotdogs are fast food.
Minor Premise: No hamburgers are hotdogs.
Conclusion: Therefore, no hamburgers are fast food.
Example of Illicit process of minor term:
Major Premise: All Paul Newman movies are great.
Minor Premise: All great movies are Oscar winners.
Conclusion: Therefore, all Oscar winners are Paul Newman movies.
In our example, the major term is “fast food”, because it is the term that appears in the major premise (first premise) as the predicate, and in the conclusion. As such, in this position, it is “undistributed”.
In our example, the minor term is “Oscar winners” because it is the term that appears in the minor premise (second premise) as the predicate, and in the conclusion. As such, in this position, it is “undistributed”.
Rule: From two universal premises no particular conclusion may be drawn
In the existential fallacy, we presuppose that a class has members when we are not supposed to do so; that is, a class has members without explicitly stating that it has members. For the conclusion to be true, at least one member of the class must exist, but the premises do not establish this. This final rule is not needed in the traditional or Aristotelian account of categorical syllogism, because that traditional account paid no attention to the problem of existential import. In modern logic, the presupposition that a class has members is seen as unacceptable.
Major Premise: All forest creatures live in the woods.
Minor Premise: All leprechauns are forest creatures.
Conclusion: Therefore, some leprechauns live in the woods.