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An untruth. An untrue statement. A wrong idea.

That which is fallacious.
The idea that the word "fallacy" has anything to do with sex is itself a fallacy.
by Athene Airheart May 15, 2004
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In logic, a fallacy is an incorrect conclusion. This may result in one of two ways.
1. If the structure of the argument is not correct at all, it is called a 'formal' fallacy. One, or both, of the first two propositions would be written incorrectly in relation to the other.
2. If the structure is correct in the first two propositions, but the conclusion is not put together properly, it is called an 'informal' fallacy.

The usual example of an argument that has no fallacy is:
Socrates is a man.
All men are mortal.
Therefore: Socrates is mortal.

Each sentence is a proposition. No propositions in an argument must be factually true to in order to create an argument with no fallacy; they must only follow the rules of logic. It is when the structure, the rules of logic, does not conform to the 256 various forms of a syllogism, it will contain a 'formal' fallacy. When only the conclusion does not follow the rules, it is an 'informal' fallacy.

Search for 'moods and figures of logic' for a full account of structure in logic.
1. It is a formal fallacy to say
Socrates is a man.
Dogs are mortal.
Therefore: Socrates is a dog. The second premise must contain either 'Socrates' or 'man' ('men), and in the right order. This is what is meant by the 'structure' of the argument.

2. It is an informal fallacy to say:
Socrates is mortal.
Dogs are mortal.
Therefore: Dogs are Socrates. The first two propositions are written correctly by the rules of logic, but the conclusion is written incorrectly.

Search for 'moods and figures of logic' for a full account of structure in logic.
by ceclark February 25, 2012
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