Guild of Historians
The historians continuously assess and re-evaluate history through the medium of today's mindset. Lord Vetinari, the Patrician of Ankh-Morpork, considers it important to let people with a certain type of mind to always be re-evaluating history, otherwise they will get their minds on something more upsetting to the general running of the city. In the current atmosphere of the city, the Guild of Historians can be a powerful political tool to Lord Vetinari, if he wants to control the behavior of, say, a man with a despised ancestor; the Historians can be persuaded to re-evaluate events in favor of the said ancestor (see Jingo).
In The Last Hero, we learn that the Guild president is currently a Mr. Betteridge.
Other named Historians include Hortensia D'Antiqua, who is credited with devising and leading walking tours which allow her to actively illustrate events in the City's history.
Assassins' Guild School History teacher Mr Linbury-Court is also a member.
The members of the Guild of Historians couldn't possibly believe that a rather magnificent War Charger of a horse had entered their Library, nor that a cowled rat could have pinched someones glasses, and so don't see these things when they do happen during the events of Thief of Time. Perhaps they truly do live in the past, or perhaps the human mind runs on such rigid rails that mere observational evidence is not proof.
The Guildhouse is on the corner of Easy Street and Treacle Mine Road, and boasts possibly the third or fourth largest reference library in the City (after the University, the Assassins' Guild and perhaps the Fools), which is open to the public. The Guild does not run a School as such, but has lecture theatres and offers day-release courses to educate students from other Guilds whose schools do not have a History faculty.
There is a certain intellectual schizophrenia involved in that at one and the same time, the Guild is required to have an appreciative objective grasp of what history actually is, as opposed to the version Lord Vetinari would like to see disseminated for public information and education.