A theory initiated by the German physicist Werner Heisenberg in 1943 and developed extensively in the 1950s and 1960s to describe strong interactions in terms of their scattering properties. S-matrix theory uses general properties, such as causality in quantum mechanics and the special theory of relativity. The discovery of quantum chromodynamics as the fundamental theory of strong interactions limited the use of S-matrix theory to a convenient way of deriving general results for scattering in quantum field theories. String theory, as a theory for hadrons, originated in attempts to provide a more fundamental basis for S-matrix theory.