 I sifted the axioms and theorems of Principia Mathematica and, on the basis of those that were found to be admissible, looked for a system of independent axioms. Given the relative completeness of Principia, this to my mind ensures the completeness of my system in the best possible way. Indeed, as a matter of principle, it is impossible to be certain that one has captured all admissible modes of inference in one formal system. (Heyting to Becker, September 23, 1933 (draft) [van Atten 2005: 129], original italics, translation mine)