Logical fallacy

Introduction

A logical fallacy is a flaw in reasoning. Logical fallacies are like tricks or illusions of thought, and they're often very sneakily used by politicians and the media to fool people. Don't be fooled! This website has been designed to help you identify and call out dodgy logic wherever it may raise its ugly, incoherent head.

Formal Fallacies

Formal fallacies, also known as logical fallacies, are flaws in the structure of an argument that render it invalid. They are errors in deductive reasoning that render an argument logically invalid, irrespective of the truth of the premises. They are often identified through the application of formal logic or a philosophical understanding of argument structure.

A group of people engaged in a heated discussion, signifying the application of logical reasoning.

Formal fallacies are categorized into two main types: propositional fallacies and quantificational fallacies. Propositional fallacies are those that involve a problem with the proposition's logic, while quantificational fallacies involve a problem with the logic of quantifiers, such as "all," "some," or "none."

Propositional Fallacies

Propositional fallacies are formal fallacies that occur in the logic of the proposition. These include:

Affirming the consequent: This fallacy occurs when a conditional statement is accepted, and the consequent is affirmed, thereby concluding that the antecedent is true. For example: "If it rains, then the ground is wet. The ground is wet. Therefore, it rained." This is a fallacy because there could be other reasons for the ground being wet.

Denying the antecedent: This fallacy occurs when a conditional statement is accepted, and the antecedent is denied, thereby concluding that the consequent is false. For example: "If it rains, then the ground is wet. It did not rain. Therefore, the ground is not wet." This is a fallacy because there could be other reasons for the ground being wet.

Quantificational Fallacies

Quantificational fallacies are formal fallacies that occur in the logic of quantifiers. These include:

Existential fallacy: This fallacy occurs when a conclusion is drawn about the existence of a thing based on a premise that quantifies over a domain that is empty or undefined. For example: "All unicorns are white. Therefore, there exists a white unicorn." This is a fallacy because the domain of unicorns is undefined or empty.

Fallacy of the undistributed middle: This fallacy occurs when the middle term in a categorical syllogism is not distributed in either the minor premise or the major premise. For example: "All dogs are mammals. All cats are mammals. Therefore, all dogs are cats." This is a fallacy because the middle term "mammals" is not distributed to all of the categories it is supposed to cover.

Informal Fallacies

Informal fallacies are errors in reasoning that occur due to the content of the arguments, rather than their logical structure. These fallacies are often identified through the application of critical thinking or a philosophical understanding of argument content.

Informal fallacies can be categorized into several types, including relevance fallacies, presumption fallacies, and ambiguity fallacies.

Relevance Fallacies

Relevance fallacies are informal fallacies that occur when the premises are not relevant to the conclusion. These include:

Ad hominem: This fallacy occurs when an argument is directed against a person rather than the position they are maintaining. For example: "You can't trust John's argument for climate change because he's a politician."

Appeal to authority: This fallacy occurs when an argument is based on the opinions of an authority figure or institution of authority, rather than on the actual argument or evidence. For example: "The government says vaccines are safe, so they must be safe."

Presumption Fallacies

Presumption fallacies are informal fallacies that occur when the conclusion is presumed in the premises. These include:

Begging the question: This fallacy occurs when the premises include the claim that the conclusion is true or (directly or indirectly) assume that the conclusion is true. For example: "God exists because the Bible says so, and the Bible is the word of God."

Complex question: This fallacy occurs when a question is asked in such a way as to presuppose the truth of some conclusion buried in the question. For example: "Have you stopped beating your wife?"

Ambiguity Fallacies

Ambiguity fallacies are informal fallacies that occur when ambiguous language is used in an argument. These include:

Equivocation: This fallacy occurs when a key term or phrase in an argument is used in an ambiguous way, with one meaning in one portion of the argument and then another meaning in another portion of the argument. For example: "The sign said 'fine for parking here', so it must be fine to park here."

Amphiboly: This fallacy occurs when the structure of a sentence allows for two different interpretations, and the argument is based on one interpretation, but the truth of the conclusion relies on the other interpretation. For example: "I shot an elephant in my pajamas. How he got into my pajamas I'll never know."