When deduction breaks: formal fallacies

Definition

Formal fallacies are deduction with the wiring crossed: the argument borrows deduction's confident shape, but the shape itself is invalid, so true premises can still deliver a false conclusion. Affirming the consequent runs an if–then backward; denying the antecedent closes one road to B and declares B unreachable; the undistributed middle links two things only through a shared category; contradictory premises argue from a pair that cannot both be true.

Memory hook

“Deduction's shape without deduction's guarantee.”

What it sounds like

• The step is wet, so it rained. (affirming the consequent)
• It didn't rain, so the step can't be wet. (denying the antecedent)
• Herons eat fish and otters eat fish, so herons are a kind of otter. (undistributed middle)
• No one was there — and they all saw it. (contradictory premises)