In logic and philosophy, a formal fallacy is a pattern of reasoning with a flaw in its logical structure (the logical relationship between the premises and the conclusion). In other words:
- It is a pattern of reasoning in which the conclusion may not be true even if all the premises are true.
- It is a pattern of reasoning in which the premises do not entail the conclusion.
- It is a pattern of reasoning that is invalid.
- It is a fallacy in which deduction goes wrong, and is no longer a logical process.
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.