Source for the Egypt part is Kronik der Frauen (Chronicle of Women) by Bodo Hareberg Verlag published circa 1990.
Ancient Egypt did one thing strangely but really well. Regardless of whether you believed the world was born from a giant cow or a praeternal ocean, Egyptian priests said it’s one and the same thing, just interpreted differently. They would have artists depict mashed up beliefs and give them a free hand in doing it. So they would (don’t remember the exact order) have a giant snake come from the praeternal ocean, who would then lay an egg, which would create the praeternal island (represented by pyramids) on which a giant cow would be born from this egg, and this cow would give birth to gods, etc.
This is the sole reason why Osiris could be his own grandfather. One myth said he was the grandkid, others that he was the grandfather so priests went “both are true”. No reason to fight over religion, everyone is happy.
This bit is based on the book by Andrzej Niwiński “Mity isymbole religijne starożytnego Egiptu”. (Mythscand religious symbols of ancient Egypt). Book is from 1984 so some things might be outdated but I think the gist is the same.
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