quality:corruption
Sequent notation

The transposition rule may be expressed as a sequent:

where ⊢ {\displaystyle \vdash } is a metalogical symbol meaning that (¬ Q → ¬ P) {\displaystyle (\neg Q\to \neg P)} is a syntactic consequence of (P → Q) {\displaystyle (P\to Q)} in some logical system; or as a rule of inference:

where the rule is that wherever an instance of " P → Q {\displaystyle P\to Q} " appears on a line of a proof, it can be replaced with " ¬ Q → ¬ P {\displaystyle \neg Q\to \neg P} "; or as the statement of a truth-functional tautology or theorem of propositional logic. The principle was stated as a theorem of propositional logic by Russell and Whitehead in Principia Mathematica as

where P {\displaystyle P} and Q {\displaystyle Q} are propositions expressed in some formal system.