Behe maintains that the complexity of light sensitivity at the molecular level and the minute biochemical reactions required for those first "simple patches of photoreceptor" still defies explanation, and that the proposed series of infinitesimal steps to get from patches of photoreceptors to a fully functional eye would actually be considered great, complex leaps in evolution if viewed on the molecular scale. Other intelligent design proponents claim that the evolution of the entire visual system would be difficult rather than the eye alone.