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Informal fallacy

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An informal fallacy is an logical argument whose premises fail to support their conclusion.[1] Informal fallacies often come about because of an error in reasoning. The premises are then incorrectly linked to the conclusion.

In contrast to a formal fallacy, the error has to do with issues of inference. Because language is used to state the propositions, natural language allows one to say more than formal logic can represent. All informal fallacies of deductive reasoning contain a disconnect between the premises and the conclusion. This makes the argument invalid. The disconnect happens because there is a hidden co-premise. If this co-premise were stated, it would make the argument valid.

Inductive informal fallacies are rather different from their deductive counterparts.[2] Their merit rests in the inductive strength of the premise–conclusion link, not the presence of hidden premises. For instance, the fallacy of hasty generalization, can be roughly stated as:

p) S is a P
p) S is also a Q
c) therefore, all Ps are also Qs

If the populations of P and Q are both too large to sample completely, then the statement is inductive. In such a case, a hasty generalization occurs when the number of Ps and Qs does not represent the respective populations.[3] It is important to distinguish between a principle of reasoning (deductive or inductive) and the premise of an argument.


  1. Kelley D. 1994. The art of reasoning. W W Norton.

    Template-specific style sheet:

    ISBN 0-393-96466-3
  2. Damer T.E. (2009). Attacking faulty reasoning: a practical guide to fallacy-free arguments (6 ed.). Wadsworth. ISBN 9780495095064 . 
  3. In other words, when you fail to mind your Ps and Qs!

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