 This meant that all the hadrons were stable particles in the first approximation, so that scattering and decays could be thought of as a perturbation.This allowed a bootstrap model with infinitely many particle types to be constructed like a field theory — the lowest order scattering amplitude should show Regge behavior and unitarity would determine the loop corrections order by order.This is how Gabriele Veneziano and many others, constructed string theory, which remains the only theory constructed from general consistency conditions and mild assumptions on the spectrum.