Consequentia mirabilis is a Latin phrase meaning ‘admirable consequence’, which refers to a law in classical logic which states that if a sentence, or proposition, follows from its own negation, then that sentence is true.
Consequentia mirabilis is expressed as the axiom: (¬φ → φ) → φ.
As an axiom, the consequentia mirabilis is a consequence of a reductio ad absurdum argument, for it can be shown that (¬φ → φ) ⊢ φ. The principle expressed is also called Clavius's Law.