Informal fallacy

An informal fallacy is an argument pattern that is wrong due to a mistake in its reasoning. In contrast to a formal fallacy, the error has to do with issues of rational inference that occur in natural language; which are broader than can be represented by the symbols used in formal logic. Informal fallacies, when deductive, commonly occur in an invalid form. By including an unstated co-premise, most deductive informal fallacies are actually valid, with the hidden co-premise false, making the argument unsound.

It is problematic to analyse inductive informal fallacies in the same way as valid or invalid, as the worthiness of an inductive argument lies in its inductive strength. For instance, the fallacy of hasty generalisation, stated as:


 * s is a P and s is a Q


 * therefore, All P's are Q's

This turns the argument into a deductive one, and in the case of a fallacy, the added premise is false. This approach tends to obliterate the distinction between induction and deduction. It is important to distinguish between a principle of reasoning (deductive or inductive) and the premise of an argument.