Front Page Titles (by Subject) INCEST. - The Works of Voltaire, Vol. V (Philosophical Dictionary Part 3)
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INCEST. - Voltaire, The Works of Voltaire, Vol. V (Philosophical Dictionary Part 3) 
The Works of Voltaire. A Contemporary Version. A Critique and Biography by John Morley, notes by Tobias Smollett, trans. William F. Fleming (New York: E.R. DuMont, 1901). In 21 vols. Vol. V.
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“The Tartars,” says the “Spirit of Laws,” “who may legally wed their daughters, never espouse their mothers.”
It is not known of what Tartars our author speaks, who cites too much at random: we know not at present of any people, from the Crimea to the frontiers of China, who are in the habit of espousing their daughters. Moreover, if it be allowed for the father to marry his daughter, why may not a son wed his mother?
Montesquieu cites an author named Priscus Panetes, a sophist who lived in the time of Attila. This author says that Attila married with his daughter Esca, according to the manner of the Scythians. This Priscus has never been printed, but remains in manuscript in the library of the Vatican; and Jornandes alone makes mention of it. It is not allowable to quote the legislation of a people on such authority. No one knows this Esca, or ever heard of her marriage with her father Attila.
I confess I have never believed that the Persians espoused their daughters, although in the time of the Cæsars the Romans accused them of it, to render them odious. It might be that some Persian prince committed incest, and the turpitude of an individual was imputed to the whole nation.
I believe that the ancient Persians were permitted to marry with their sisters, just as much as I believe it of the Athenians, the Egyptians, and even of the Jews. From the above it might be concluded, that it was common for children to marry with their fathers or mothers; whereas even the marriage of cousins is forbidden among the Guebers at this day, who are held to maintain the doctrines of their forefathers as scrupulously as the Jews.
You will tell me that everything is contradictory in this world; that it was forbidden by the Jewish law to marry two sisters, which was deemed a very indecent act, and yet Jacob married Rachel during the life of her elder sister Leah; and that this Rachel is evidently a type of the Roman Catholic and apostolic church. You are doubtless right, but that prevents not an individual who sleeps with two sisters in Europe from being grievously censured. As to powerful and dignified princes, they may take the sisters of their wives for the good of their states, and even their own sisters by the same father and mother, if they think proper.
It is a far worse affair to have a commerce with a gossip or godmother, which was deemed an unpardonable offence by the capitularies of Charlemagne, being called a spiritual incest.
One Andovere, who is called queen of France, because she was the wife of a certain Chilperic, who reigned over Soissons, was stigmatized by ecclesiastical justice, censured, degraded, and divorced, for having borne her own child to the baptismal font. It was a mortal sin, a sacrilege, a spiritual incest; and she thereby forfeited her marriage-bed and crown. This apparently contradicts what I have just observed, that everything in the way of love is permitted to the great, but then I spoke of present times, and not of those of Andovere.
As to carnal incest, read the advocate Voglan, who would absolutely have any two cousins burned who fall into a weakness of this kind. The advocate Voglan is rigorous—the unmerciful Celt.