[voice]
Ah, but you are thinking there only of matter, and not of the differences that may exist between the energy of a living soul and the energy held within a rock. In Soul Society, all things are made up of spiritual particles called reishi, from the people of Rukongai to the smallest speck of dust, and yet, when a human soul enters Soul Society from the world of the living, it cannot be placed inside the body of a fish.