I guess this would hinge on the notion that if something has potentials that means its actuality is being determined by something in act. One could approach this a number of ways. If the supposed prime mover has potential parts which limit its being, we would need to ask why it has those potentials in the first place, what is limiting its being? For instance, the potential for a chair to be stationed at point A or C limits it to to being a chair that is stationed at point B. But, in part, this limitation is due to something which is determining is actuality to actualize the potential of existing at point B. So, given the PC, we need to mount up the causal chain to find that which determines its actuality to exist in one specific way instead of the other potential ways--that which determines how actuality is divided among potentials.