The Torah, “Leviticus” (Tzav), 6:3-6:4: And the Cohen (priest) shall don his linen tunic, and he shall don his linen trousers on his flesh. And he shall lift out the ashes into which the fire has consumed the burnt offering upon the altar, and put them down next to the altar. He shall then take off his garments and put on other garments, and he shall take out the ashes to a clean place outside the camp.
Ash symbolizes the burnt part of our desire that we cannot correct (the fourth part, Behina Dalet). And so it is taken out of the camp because it is impossible to use it.
The camp refers to the four levels of our ego, which include the elderly, the women, the children, the Levites, and the Cohanim (priests). No one is allowed to use the heaviest ego that cannot be corrected. It remains part of nature, but cannot be used and is therefore taken away.
It is similar to the custom of throwing part of the skin after the circumcision to the sand and then totally getting rid of it. This means that the parts that we cannot use symbolize the greatest egoistic levels that cannot be corrected since we don’t have the Masach (screen) for that, the powers to resist.
From KabTV’s “Secrets of the Eternal Book” 12/6/13