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).
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.
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.