The bible of Jewish mysticism, the 13th century AD pseudepigraphal work called the Zohar (Splendour), expressly forbids anyone from imagining how the Hebrew god might have looked prior to his magicing this world into existence when he took on the form of man (2:42b): "it is forbidden to one who apprehends Him as He is before creation to picture Him under any form or shape whatsoever".
Seven centuries after the Zohar was written, a Jewish historian named Léon Poliakov was one of two chief scribes of a not too dissimilar injunction, contained within a letter published in Le Monde on February 21, 1979, bearing the title "Hitler's extermination policy: a statement by French historians" (although several of them were Jewish and were not born in France):
One must not ask oneself how, technically, such a mass-murder was possible. It was technically possible, since it happened. That is the requisite starting point of any historical inquiry into the subject. It is incumbent upon us to state this truth simply: there is not, there cannot be any debate on the existence of the gas chambers.
This text was signed by historians whose names follow and who work or teach at the College de France, CNRS, the universities of Paris and the provinces, the Ecole des Hautes Etudes en Sciences Sociales, at the Ecole Pratique high school: Philippe Ariès, Alain Besançon, Robert Bonnaud, Fernand Braudel, Pierre Chaunu, Monique Clavel-Levêque, Marc Ferro, François Furet, Yvon Garlan, Jacques Julliard, Ernest Labrousse, Jacques Le Goff, Emmanuel Le Roy Ladurie, Pierre Levêque, Nicole Loraux, Robert Mandrou, Claude Mossé, Roland Mousnier, Jacques Néré, Claude Nicolet, Valentin Nikiprowetzky, Evelyne Patlagean, Michelle Perrot, Léon Poliakov, Madeleine Rebérioux, Maxime Rodinson, Jean Rougé, Lilly Scherr, Pierre Sorlin, Lucette Valensi, Jean-Pierre Vernant, Paul Veyne, Pierre Vidal-Naquet, Édouard Will.
The full text of all the articles appearing on the above page can also be read here. I've taken the English translation from here, a very interesting article consisting of Robert Faurisson's letters published in Le Monde, which compelled the 34 historians to issue their ridiculous prohibition.
If any French speakers fancy translating any interesting snippets from these articles, I'd be very grateful.