The Book of Knowledge once told him that to distinguish whether a soul is an ancient evil thought, there is only one way: dissect the soul in a frozen state.

Of course, this would also lead to the soul's complete death.

Unless it's absolutely certain that it is the reincarnation of an ancient evil thought, this method should not be used.

The large-scale mutual suspicion among humans, leading to internal strife, is also one of the means by which doomsday cultists undermine the power of fire.