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A formal fallacy is contrasted with an informal fallacy which may have a valid logical form and yet be unsound because one or more premises are false. A formal fallacy, however, may have a true premise, but a false conclusion. The term 'logical fallacy' is sometimes used in everyday conversation, and refers to a formal fallacy.
Logical Fallacies, Literacy Education Online; Informal Fallacies, Texas State University page on informal fallacies; Stephen's Guide to the Logical Fallacies (mirror) Visualization: Rhetological Fallacies, Information is Beautiful; Master List of Logical Fallacies, University of Texas at El Paso; Fallacies, Internet Encyclopedia of Philosophy
A formal fallacy, deductive fallacy, logical fallacy or non sequitur (Latin for "it does not follow") is a flaw in the structure of a deductive argument that renders the argument invalid. The flaw can be expressed in the standard system of logic. [1] Such an argument is always considered to be wrong.
These two definitions of formal logic are not identical, but they are closely related. For example, if the inference from p to q is deductively valid then the claim "if p then q" is a logical truth. [16] Formal logic needs to translate natural language arguments into a formal language, like first-order logic, to assess whether they are valid.
Fallacies are usually divided into formal and informal fallacies. Formal fallacies are expressed in a formal language and usually belong to deductive reasoning. Their fault lies in the logical form of the argument, i.e. that it does not follow a valid rule of inference. [98] [99] A well-known formal fallacy is affirming the consequent.
The fallacy of four terms is a syllogistic fallacy. Types of syllogism to which it applies include statistical syllogism, hypothetical syllogism, and categorical syllogism, all of which must have exactly three terms. Because it applies to the argument's form, as opposed to the argument's content, it is classified as a formal fallacy.
The only situation where one may deny the antecedent would be if the antecedent and consequent represent the same proposition, in which case the argument is trivially valid (and it would beg the question) under the logic of modus tollens. A related fallacy is affirming the consequent.
Formal arguments that are invalid are often associated with at least one fallacy which should be verifiable. A standard view is that whether an argument is valid is a matter of the argument's logical form. Many techniques are employed by logicians to represent an argument's logical form.