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Freedom of speech is understood to be fundamental in a democracy. What limitations to freedom of speech are there in France? Is holocaust denial an example? can one insult prophets?
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The limitations to freedom of speech in France are very similar to most western countries when it comes to the following (all forbidden) themes:
incitation to racial hatred and violence
denial of some historically established genocides like the holocaust or the armenian genocide.
injure : more or less insult not supported by any evidence
...and probably others that I don't know of. Some people regularly get fines or jail times for racial hate speech or holocaust denial. In many aspects, the laws against racial insults and hatred, even racism based on religion, are actually harsher than in most western countries. Using the very word race in public is reprehensible. Besides, ethnic and religious statistics are forbidden. No one knows how many catholics/atheists/muslims/jews/... there are in France, as well as black/white/asian/arabic/indian/... people. However, a religion can be insulted at will. Here is why.
The first article of the constitution starts as follows : "La France est une République indivisible, laïque, démocratique et sociale. Elle assure l’égalité devant la loi de tous les citoyens, sans distinction d’origine, de race ou de religion. Elle respecte toutes les croyances." This translates as "France is an undividable, laïque, democratic and social republic. It ensures equality of all its citizens before the law, without any regard to origins, race, or religion. It respects every belief. "
What foreigners don't get about this is the term laïque, which cannot really be translated (secular does not really have the right tone).
The separation of church and state law from 1905 defines the term laïque says "la République ne reconnaît, ne salarie, ni ne subventionne aucun culte." This can be translated as "the republic does not recognise or finance any religion/cult". In other terms, according to the French law, no religion EXISTS, they are completely ignored.
This means that you can insult any religion or any prophet as much as you want. If you want a good example, Charlie Hebdo's covers (mostly about catholicism) are quite provocative, and yet not condemned by law (some could have been but not on the basis of insult to religion, which does not exists at all). Religious french people just have to not feel insulted whenever their religion is insulted. Just like when someone curses the wind.
Some protection they have by law are:
Insulting the people who follow a religion is condemned as racial hate speech.
Insulting anybody because of his/her religion is condemned as racist insult
Vandalising any worship place is condemned as any vandalisation.
I will add that the constitution articles quoted in the other answer (10 and 11) are pretty clear : freedom of religion is respected WITHIN the law. This is based on the reciprocity principle : your freedom stops where the one of others begin.
Misunderstandings with anglo-saxons (and more generally foreigner) observers occurs when they think they understand everything there is to know about France by picturing the same system, laws and political culture than they have except for certain points. It is not.
Side remark : "Charlie Hebdo" comes from a small sub-culture of France that I did not encounter anywhere else. But this is another topic.
Article 10 of the french Déclaration des Droits de l'Homme et du Citoyen in 1789 (still applied):
Art. 10. -
Nul ne doit être inquiété pour ses opinions, même religieuses, pourvu que leur manifestation ne trouble pas l'ordre public établi par la Loi.
Art. 11. -
La libre communication des pensées et des opinions est un des droits les plus précieux de l'Homme : tout Citoyen peut donc parler, écrire, imprimer librement, sauf à répondre de l'abus de cette liberté dans les cas déterminés par la Loi.
which can be translated to : Nobody should fear for his opinions, including religious ones, provided that their expression doesn't endanger the public order. An interesting part of the 11th article is the "abuse of that freedom" part.
I don't know what stands for denial. @talha-irfan already started on Charlie Hebdo.
One famous exception to freedom of speech in France is inciting violence and hatred.