 Despite the fact that Heisenberg relied heavily on the correspondence principle in developing matrix mechanics, he no longer publicly defended  it.[9]  Indeed as Mara Beller has noted, “Heisenberg does not cite Bohr’s work at all [in the 1925 matrix mechanics paper], despite the fact that the paper was built on Bohr’s correspondence principle in a fundamental way… Instead Heisenberg [inaccurately] presented his work as flowing from the positivist principle of elimination of unobservables” (Beller 1999, p.  140).[10]  Heisenberg expresses his increasing ambivalence towards the correspondence principle in his book The Physical Principles of the Quantum Theory: