Then, whenever " P → Q {\displaystyle P\to Q} " and " ¬ Q {\displaystyle \neg Q} " each appear by themselves as a line of a proof, then " ¬ P {\displaystyle \neg P} " can validly be placed on a subsequent line.
Kemudian, setiap kali " P → Q {\displaystyle P\to Q} "dan " ¬ Q {\displaystyle \neg Q} "masing-masing muncul dalam pembuktian, kemudian " ¬ P {\displaystyle \neg P} " secara sah dapat ditempatkan pada kesimpulan.