One question people often ask is why the university is called “Paris 7”, followed by the realization that there are (were!) thirteen universities in Paris, numbered from “Paris 1” to “Paris 13”. Here’s an attempt at explaining it (though I’m sure I can’t cover all the reasons.)
At the beginning
Prior to the 70’s, there was one big Parisian university, called Université de Paris. It was founded in 1896 and was the successor of the university founded in the XIIth century which closed down during the French revolution. Its headquarters were in the “Sorbonne” building, and it was common to simply call the university “la Sorbonne”, a name that is still rather famous. It was subdivided in “faculties”, such as the science faculty or the law faculty.
While higher education used to be reserved for the elite, after the two world wars, more and more students started to go to university. According to Wikipedia, while in 1914 there were about 17,000 students, in 1956 there were 64,000. (As a comparison, today the Parisian universities total more than 330,000 students.) This led to the expansion of the university in the suburbs of Paris and in new campuses around the city.
Then in May 1968 (popularly known as mai soixante-huit), France knew a period of unrest. Students started to strike in Paris, to protest against capitalism, consumerism, American imperialism, and de Gaulle. The president of the university called the police to suppress the strike, which did not go well. Some professors started supporting the students and striking too. Students erected barricades to block streets, protests spread around France, and in the middle of May, about half a million people demonstrated through Paris. This was followed by a general strike, during which every French worker was invited to go on strike; on May 22nd, ten million people did not go to work.
After a bit of flailing around, at the end of May, the government signed the “Grenelle Agreements”. These included, among other things, a 35% augmentation of minimum wage, a 10% increase of all wages, the creation of company chapters for unions, and a fourth week of paid holidays for all workers. President De Gaulle also decreed the dissolution of the National Assembly (the main legislative body of France), followed by elections which were largely won by De Gaulle’s party. He also announced a referendum on a constitutional reform at that time, which took place a year after. He lost that referendum, had to resign, and was replaced by Pompidou. May 68 is still largely present in French culture (although it has been… sugarcoated in collective memory and mediatic renditions). Some slogans are still very well known, such as the rhyming “CRS: SS”, where CRS is the French riot police, and SS is the Nazi political police. The legends tell us that CRS’s chanted back “étudiants: diants diants”, where “étudiants” means “students” and “diants”… is meaningless.
Following this eventful month, a reform of French higher education was seen as necessary. In 1970, the government invited professors to divide themselves in the newly created Teaching and Research Units (UFR) that replaced the former faculties, while the UFRs themselves grouped into new universities. Rivalries and office politics apparently played a large role. On December 31st, thirteen new universities were created out of the old University of Paris. Some of them were created based on geographic criteria (e.g. Paris-XI is Paris-South in Orsay, a suburb at the south of Paris; Paris-XIII is Paris-North in Villetaneuse, another suburb in the north; and so on). The ones in the center of Paris were created mostly based on academic subdivisions. The main examples you may know, since you are probably mathematicians, are probably Paris-VI (Pierre et Marie Curie, that you may also know as “Jussieu”) and Paris-VII (Diderot) – the first one being dedicated to sciences and medicine, the second to sciences, literature & social sciences, and medicine.
Almost all French universities above a certain size were split in a similar way. Since none were as big as the university of Paris, the “traditional” split was to create three universities: science/technology, law/medicine, literature/language/art/social sciences.
Of course, this is not the end of the story. Some time ago, it was realized that French universities do not perform very well in international rankings of universities – you know the ones. There are several reasons that French universities are not ranked well, but an important one is that the criteria used by these rankings are often absolute numbers, not compared to the number of students of the university, which led to some absurdities. The political power took this very seriously. In fact, in 2007, president Sarkozy addressed a letter of mission to Mrs. Pécresse, the ministry of Higher Education and Research, expressly asking her to “improve the ranks of [French] higher education centers in international rankings, with the objective of getting at least two French establishments in the top 20 and ten in the top 50”.
It was thus discovered that universities can merge back to increase their rank (which in itself is indicative of the quality of these rankings). The governement created huge multi-annual grants called IDEX, worth close to a billion euros each. The criterion used is essentially the performance of the university in these international rankings. All universities competed (and still compete) for these grants, and for a new kind of grant called I-SITE, which has the same basic function but of a smaller caliber (a third to half a billion euros).
Fast forward to today, all universities in France are merging back again in the hopes of being awarded these grants. In Lille (where I did my doctorate) this already happened, and the three universities (Lille-I, where I worked, Lille-II, and Lille-III) now form the University of Lille. In Paris this is not so simple, as simply merging everything would be a huge incoherent administrative mess. So some universities are merging with one another to create a few big Parisian universities. This already happened to Paris-VI (“Jussieu”): it merged with Paris-IV (known as Paris-Sorbonne) to create “Sorbonne Université”. Paris-VII (Paris-Diderot, the one I am going to) will merge in January 2019 with Paris-V (known as Paris-Descartes), as well as the IPGP, to create a new big university with a yet unknown name. So my affiliation in about six months will be a bit different…