A restricted axiom scheme in the language of arithmetic due to mathematician Andrei Markov, Jr. (1903–1979) of the form:
when is decidable. On the BHK interpretation of mathematics, this is read as the statement that whenever the inexistence of a witness for a decidable would lead to absurdity, one is able to construct such a witness. Markov’s principle is not valid in Heyting Arithmetic (), although the rule form of the axiom (Markov’s rule) is admissible in .