‘Meanwhile Heisenberg pursued some work of his own, keeping its idea and purpose somewhat dark and mysterious. Towards the end of the summer semester, in the first days of July 1925, he came to me with a manuscript and asked me to read it and decide whether it was worth publishing … He added that though he had tried hard, he could not make any progress beyond the simple considerations contained in his paper, and he asked me to try myself, which I promised …

His most audacious step consists in the suggestion of introducing the transition amplitudes of the coordinates q and momenta p in the formulae of mechanics …

I was most impressed by Heisenberg's considerations, which were a great step forward in the programme which we had pursued …

After having sent Heisenberg's paper to Zeitschrift für Physik for publication, I began to ponder about his symbolic multiplication, and was soon so involved in it that I thought the whole day and could hardly sleep at night. For there was something fundamental behind it … And one morning … I suddenly saw the light: Heisenberg's symbolic multiplication was nothing but matrix calculus, well known to me since my student days from the lectures of Rosanes at Breslau.’