Formal vs informal fallacies a fallacy is a defect in an argument other than its having false premises An informal fallacy is a defect in the content of an argument.(A formal fallacy is a defect in the structure of an argument. We have seen many valid rules of deduction Formal fallacies can be understood as a use of unacceptable rules2 Formal Vs Informal Fallacies • A fallacy is a defect in an argument other than its having false premises. • An informal fallacy is a defect in the content of an argument. (A formal fallacy is a defect in the structure of an argument.) • We have seen many valid rules of deduction. Formal fallacies can be understood as a use of unacceptable rules