Informal fallacy

An informal fallacy is an logical argument whose  premises fail to support their conclusion.Kelley D. 1994. The art of reasoning. W W Norton.  Informal fallacies often come from errors in reasoning.

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. Their merit rests in the inductive strength of the premiseconclusion 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. It is important to distinguish between a principle of reasoning (deductive or inductive) and the premise of an argument.

Some informal fallacies are fallacies of ambiguity. For example:

 Feathers are light. ("light" as "not heavy")
 What is light cannot be dark. ("light" as "containing a lot of white")
 Therefore, feathers cannot be dark.

Equivocations are more difficult to detect in cases where the two meanings are closely related to each other.

References

Other websites
 Logical fallacies A list of logical fallacies, explained.
 An Informal Fallacy Primer 

Category:Logical fallacies
Category:Critical thinking