Deduction draws conclusions that must be true if the premises are true.
1Valid forms
If P then Q; P ⇒ Q (modus ponens)
If P then Q; not Q ⇒ not P (modus tollens)
P or Q; not P ⇒ Q (disjunctive syllogism)
If P then Q, if Q then R ⇒ if P then R (chain)
2Fallacies
If P then Q; Q ⇒ P is invalid (affirming the consequent)
If P then Q; not P ⇒ not Q is invalid (denying the antecedent)