  Various pieces of insight were needed to formulate the rules for working with
gauge theories of the electro-magnetic, weak and strong forces. First, it was
needed to understand how to formulate the Feynman rules. We had to learn that
there are many different ways to derive them, and it was needed to know how
different formulations of the gauge constraint lead to the same final results:
the calculated values of the scattering amplitudes. The rules for dealing with
the infinities that had to be subtracted were a big challenge, culminating in
the discovery of the Becchi-Rouet-Stora-Tyutin symmetry. Fond recollections of
the numerous discussions the author had with Raymond Stora on this topic are
memorised here. We end with some reflections on the mathematical status of
quantum field theories, and a letter sent by Stora to the author
