Inflation as the Unique Causal Mechanism for Generating Density Perturbations on Scales Well Above the Hubble Radius

An examination is made of the widely held belief that inflation is the only possible causal mechanism capable of generating density perturbations on scales well in excess of the Hubble radius. A simple proof is given, which relies only on the assumption that our understanding of the universe from nucleosynthesis onwards is correct. No assumption of the underlying gravitational theory is necessary beyond that it is a metric theory.