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Formal Fallacies - by Mike Moum  

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15/07/2018 3:53 pm  

Logical fallacies come in two forms: formal and informal. This post will discuss formal fallacies in general terms.

A formal fallacy is one which involves an error in the form of an argument. The question in view is not whether a conclusion is true or false, but whether the form of the argument is correct or incorrect, valid or invalid.

The concluding statement of an argument may be objectively true, though the argument is formally invalid; or the concluding statement may be objectively false, though the argument is formally valid.

Here's an example of true conclusion but invalid argument:
P1: Some men are mortal.
P2: Socrates is a man.
C1: Therefore Socrates is mortal.
It is true that Socrates is mortal. The problem is that P1 allows for the possibility of men who are not mortal, so it is possible that Socrates is one of those men, hence we cannot conclude that he is in the group of mortal men.

Here is an example of a valid argument with a false conclusion:
P1: All men are green.
P2: Socrates is a man.
C1: Therefore Socrates is green.
The conclusion is false because the first premise is false, but the form of the argument is correct.

And here is an argument that is both invalid and has a false conclusion:
P1: Some men are green.
P2: Socrates is a man.
C1: Therefore Socrates is green.
This argument suffers from the same formal problem is the one immediately above, and additionally, has the false premise that some men are green.

For a more thorough discussion of formal fallacies, the Wikipedia articles is excellent:  https://en.wikipedia.org/wiki/Formal_fallacy


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