By the late 1950s, many strongly interacting particles of ever higher spins had been discovered, and it became clear that they were not all fundamental.
In 1967, a notable step forward in the bootstrap approach was the principle of DHS duality introduced by Richard Dolen, David Horn, and Christoph Schmid in 1967,[REF] at Caltech (the original term for it was "average duality" or "finite energy sum rule (FESR) duality").