Musar su Salmi 22:21

And, likewise, one who is accustomed to eat unkosher meat or to feed it to a Jew. And this is intimated in the Torah, it being written (Shemoth 23:1): "And men of holiness shall you be to Me, and flesh torn in the field you shall not eat. To the dog shall you throw it," after which it is written (Ibid. 23:1): "You shall not spread a false report." It emerges, then, that the verse of "To the dog shall you throw it" is found between these two sins — eating treifah [torn flesh] and speaking lashon hara, to teach us that for these two sins, a man may be reincarnated as a dog. And this is what King David, may peace be upon him, intimated in (Psalms 22:21): "Rescue from the sword, my soul; [rescue] from the dog, my soul," followed by "I will speak Your name to my brothers, etc." That is, I do not use my tongue to speak lashon hara, to be punished therefor by this dreadful reincarnation, but I use it to praise You and to exhort Israel, that they should fear You and praise You. And know that the Kabbalists have said that even though when a man is reincarnated in the form of another man he is unaware of his prior state, still when he is reincarnated as an animal or as a bird, he is aware of his prior state and suffers terribly at having descended from heaven from the form of a man to the form of a beast. Therefore, every man should fear and tremble and be soft of heart while he yet lives, while he yet has free will and knows his G-d, so that He forgives his sins and removes His wrath from him. And when his soul leaves him he will rest in peace and repose in His shade in Gan Eden. For He is gracious and merciful and abundant in lovingkindness. And (Berachoth 34b): "In the place where penitents stand, absolute Tzaddikim cannot stand." Until here, the words of the Sefer Charedim, in short.

We can better understand the reason that the Torah permits Edomites and Egyptians (of the third generation after conversion) to become fully fledged members of the Jewish people if we take a look at the historical development of Abraham, the first convert to Judaism. Both the second generation after Abraham, i.e. Isaac-Ishmael, and the third generation, i.e. Jacob-Esau, still contained a considerable amount of פסולת, spiritual refuse. Only the fourth generation of Abraham's descendants, Jacob's sons, were completely pure and had divested themselves of their spiritual waste. Esau actually became the means that Jacob received the blessing. As a recognition of this the third generation of converts descended from Esau/Edom are admitted as full fledged members. The Torah acknowledges this in 23,8: "Do not despise Edom for he is your brother;" this means that after living as a Jew for the third generation Esau once more qualifies as a brother of Jacob not only biologically but also spiritually. The same law applies to the absorption of Egyptians into the community of the Jewish nation. Our exile in Egypt actually commenced at the time G–d foretold Abraham about it at the covenant of the pieces in Genesis 15,13. Abraham was told that the fourth generation of his descendants would return to the land of Canaan. While in Egypt, the souls of the Jewish people were refined to the extent that they were able to become a nation comprising over 600,000 men of military age, a people that qualified for the title "Israel," the honorific title bestowed by G–d on Jacob. It is appropriate therefore that the third generation of an Egyptian convert to Judaism should enjoy full equality with other Jews just as Jacob who was the third generation counting from Abraham's conversion, was pure and had shed all the spiritual פסולת which had remained from the pollutant of the serpent. Israel, too, had remained גרים, strangers, in Egypt for three generations. The prohibition of bringing the wages of harlotry to the altar of G–d is explained simply by the fact that the harlot is beholden to Lilith, the antithesis of the שכינה. It is a domain where the dogs cry out because the angel of death is constantly around. This is why David prayed in his Psalm 22,21: "Save my life from the sword, my precious life from the clutches of a dog."