The Structure of Deductive Arguments

Valid deductive arguments follow strict logical forms:

• Syllogism — two premises lead to a conclusion. All A are B, C is A, therefore C is B
• Modus ponens — if P then Q. P is true. Therefore Q is true
• Modus tollens — if P then Q. Q is false. Therefore P is false
• Disjunctive syllogism — either P or Q. Not P. Therefore Q

An argument can be logically valid (the conclusion follows from the premises) yet unsound (a premise is false). Validity and truth are independent: both must hold for deduction to give a reliable answer.