Pfeifer and coworkers claim that the "division of the total energy–momentum tensor into electromagnetic (EM) and material components is arbitrary". In other words, the EM part and the material part in the total momentum can be arbitrarily distributed as long as the total momentum is kept the same. But Mansuripur and Zakharian don't agree, and they suggested a Poynting vector criterion. They say for EM radiation waves the Poynting vector denotes EM power flow in any system of materials, and they claim that the Abraham momentum is "the sole electromagnetic momentum in any system of materials distributed throughout the free space".