Logical fallacies are errors that occur in arguments. In logic, an argument is the giving of reasons (called premises) to support some claim (called the conclusion). Arguments may be classified as

*deductive *or

*inductive*. Deductive arguments assert or imply that the conclusion follows

*necessarily *from the premises. Inductive arguments assert or imply that the conclusion follows with

*some degree of probability*, not necessity. Deductive arguments are evaluated for validity. If the conclusion of a deductive argument follows necessarily from the premises, the argument is said to be

*valid*. If the conclusion of a deductive argument does

*not *follow with necessity from the premises, the argument is said to be

*invalid.* Validity is determined by the

*form *of the argument, not the truth or falsity of the premises or the conclusion. An argument with the form

*If p then q; p; so, q *is a valid argument, no matter what statements are represented by

*p* and

*q*. If

*p* and

*if p then q* are both true, then

*q *must be true. An argument with the form

*If p then q; q; so p* is invalid no matter what statements are represented by

*p* and

*q*. Even if

*q* and

*if p then q* are true,

*p* is not necessarily true. (Note: to say a statement is

*not necessarily true* is not the same as saying that it is false.) The invalid argument form just presented is called

*affirming the consequent* and is known as a

*formal fallacy*. Inductive arguments may be evaluated by their form, but usually they are evaluated by other criteria. The fallacies of induction are called

*informal* fallacies because they do not evaluate the form to determine validity. I'll go over the criteria for a cogent inductive argument as I discuss the informal fallacies below.