"I tried at that time to cast the unifying Dirac-Jordan transformation theory into a simpler and more easily understandable form and to convey its essence to Hilbert. When von Neumann saw this he cast it in a few days into an elegant axiomatic form much to the liking of Hilbert. (This is the origin of the paper “Ober die Grundlagen der Quantenmechanik” by Hilbert, von Neumann and myself . . .). The method used was that of integral operators. . . . This work set von Neumann on his way to his definite studies on the foundations of quantum mechanics."