I kind of liked one of the HPMOR recursive fanfics which played with this since in the Methodsverse wizards continued to be able to think even when their brain was replaced with that of a cat.
If I remember right in that fic the blood-purists maintained that only wizards had souls and that the thing they used to prove it was by feeding polyjuice to a non-wizard who would lose their own personality in favor of the body they were copying for the duration of the potions effects. (not having a magical soul with which to maintain their thought patterns when their physical brain was changed)
I kind of liked one of the HPMOR recursive fanfics which played with this since in the Methodsverse wizards continued to be able to think even when their brain was replaced with that of a cat.
If I remember right in that fic the blood-purists maintained that only wizards had souls and that the thing they used to prove it was by feeding polyjuice to a non-wizard who would lose their own personality in favor of the body they were copying for the duration of the potions effects. (not having a magical soul with which to maintain their thought patterns when their physical brain was changed)