Fallacy: An error in reasoning.

Formal Fallacy: An error (invalid deductive inference) resulting from the structure of an argument. Some examples include the fallacy of Affirming the Consequent.

We’ve already experienced some examples of formal fallacies:

If A then B. Some A are B

B Some A are C

Therefore A Some A are both B & C.

Informal Fallacy: An error in reasoning where the error cannot be detected except by considering the content (i.e. meaning) of the statements in the argument.

Two kinds of Informal Fallacy:

Fallacies of Relevance: An error in reasoning where the premise in an argument are irrelevant to the conclusion of the argument.

Fallacies of Insufficient Evidence: An error in reasoning where the premises are typically relevant, but nonetheless are insufficient to support the conclusion.

Relevance: A statement (e.g. a premise) is only relevant (or irrelevant) relative to another statement. For a statement to be relevant, the truth of that statement (i.e. the premise) must make a difference insofar as the truth or falsity of another statement (i.e. the conclusion).

Positive Relevance: A statement is positively relevant when the truth of the statement (i.e. premise) makes some other statement (i.e. conclusion) more likely to be true. Though, this need not imply the inference is strong, namely that the conclusion is made likely, only that the conclusion is made more likely.

Negative Relevance: A statement is negatively relevant when the truth of the statement (i.e. premise) makes some other statement (i.e. conclusion) less likely to be true.

Irrelevance: A statement is irrelevant when the truth of the statement (i.e. premise) make no difference whatsoever to the likelihood that another statement (i.e. the conclusion) is true.

Consider the claim, “Chris is a woman.”

This claim is positively relevant to the claim, “Chris is a mother.” Though it is insufficient to prove that the statement is true, it does make the statement more likely than it would be if it were false that “Chris is a mother.”

This claim is negatively relevant to the claim, “Chris plays linebacker for the Raiders.”

This claim is irrelevant to the claim, “Chris is a graduate student.”