But when Celsus speaks of heroes and demons, he starts a deeper question than he is aware of. For after the statement which he made in regard to service among men, that “the first master is injured when any of his servants wishes at the same time to serve another,” he adds, that “the same holds true of heroes, and other demons of that kind.” Now we must inquire of him what nature he thinks those heroes and demons possess of whom he affirms that he who serves one hero may not serve another, and he who serves one demon may not serve another, as though the former hero or demon would be injured in the same way as men are injured when they who serve them first afterwards give themselves to the service of others.
Let him also state what loss he supposes those heroes or demons will suffer. For he will be driven either to plunge into endless absurdities, and first repeat, then retract his previous statements; or else to abandon his frivolous conjectures, and confess that he understands nothing of the nature of heroes and demons. And in regard to his statement, that men suffer injury when the servant of one man enters the service of a second master, the question arises: “What is the nature of the injury which is done to the former master by a servant who, while serving him, wishes at the same time to serve another?”
Source: Against Celsus (New Advent)