"Thus much I can declare of the Persians with entire certainty, from my own actual knowledge. There is another custom which is spoken of with reserve, and not openly, concerning their dead. It is said that the body of a male Persian is never buried, until it has been torn either by a dog or a bird of prey. That the Magi have this custom is beyond a doubt, for they practice it without any concealment. The dead bodies are covered with wax, and then buried in the ground. The Magi are a very peculiar race, different entirely from the Egyptian priests, and indeed from all other men whatsoever. The Egyptian priests make it a point of religion not to kill any live animals except those which they offer in sacrifice. The Magi, on the contrary, kill animals of all kinds with their own hands, excepting dogs and men. They even seem to take a delight in the employment, and kill, as readily as they do other animals, ants and snakes, and such like flying or creeping things. However, since this has always been their custom, let them keep to it. Buying and selling in a marketplace is a custom unknown to the Persians, who never make purchases in open marts, and indeed have not in their whole country a single market-place." (Encyclopedia of Ancient History).
Herodotus on Ancient Persian Notion of Gods
