Logical Fallacies (Formal)
Some logical errors are FORMAL — meaning they have flawed STRUCTURE regardless of content. Two famous examples involve conditional statements ("if P then Q"). AFFIRMING THE CONSEQUENT: "If it rains, the streets are wet. The streets are wet. Therefore it rained." But streets could be wet for other reasons (sprinklers, etc.). DENYING THE ANTECEDENT: "If it rains, the streets are wet. It did not rain. Therefore the streets are not wet." Same problem.
Valid forms vs fallacies. MODUS PONENS (valid): "If P then Q. P is true. Therefore Q." This works. MODUS TOLLENS (valid): "If P then Q. Q is false. Therefore P is false." Also works. AFFIRMING the CONSEQUENT (invalid fallacy): "If P then Q. Q is true. Therefore P." Does not follow. DENYING the ANTECEDENT (invalid fallacy): "If P then Q. P is false. Therefore Q is false." Also does not follow. Knowing which form is which prevents these errors.
"All birds have feathers. This animal has feathers. Therefore it is a bird." This is which fallacy?
Why these matter. Many real-world arguments commit these fallacies invisibly. Pseudoscience often does. So do conspiracy theories. So do well-meaning explanations. Knowing the FORM helps you spot bad reasoning even when the content sounds plausible. With practice, fallacy detection becomes automatic. Formal logic equips you with a mental "fallacy filter."
Spot Three
In the next week, find 3 real-world arguments. For each, write out: premises, conclusion, and structure. Did the conclusion follow validly? Most arguments in casual conversation do not — and yet sound convincing.
Formal fallacies are reliable bad-reasoning patterns. Knowing them sharpens thinking AND makes your own arguments more rigorous.
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