Von Neumann's psycho-physical parallelism requires the existence of an interaction between subjective experiences and material systems. A hypothesis is proposed that amends physics in a way that connects subjective states with physical states, and a general model of the interaction is provided. A specific example shows how the theory applies to pain consciousness. The implications concerning quantum mechanical state creation and reduction are discussed, and some mechanisms are suggested to seed the process. An experiment that tests the hypothesis is described elsewhere. Key Words: von Neumann, psycho-physical, consciousness, state reduction, state collapse, macroscopic superpositions, conscious observer.