... Parisi and Sourlas... observed that the most infrared divergent diagrams are those with the maximum number of random source insertions, and, if the other diagrams are neglected, one is left with a diagrammatic expansion for a classical field theory in the presence of random sources. Parisi and Sourlas then pointed out that the underlying phenomenon for the connection between random systems and pure systems in two fewer dimensions is that a classical field theory in the presence of random sources is perturbatively equivalent to the corresponding quantum field theory in two fewer dimensions. Parisi and Sourlas explained this dimensional reduction by a hidden supersymmetry.