An ye harm none do as ye will - this includes any living creature and it means in thought, word, deed and intent. Therefore power or magic used to harm will be sent back to the sender. What is done will be returned to the doer. Some believe t hat it is the Goddess or the God who performs this function.
Human souls enjoy a series of incarnations in human form.
How and why is open to mystical speculation. There are no specific teachings regarding this doctrine. Some say we never change sexes, some say we choose whatever gender is appropriate for the lessons of that particular lifetime. Power can be sent in non-physical form to affect the world in positive ways. Thus we accept both the practice of magic and its effectiveness.