AI-based surveillance, which became the basis of an exact historical record-taking, and later of simulation of conditions for which there were not any direct records.
Cliology is the study of history taken to the next level. With the coming of advanced information technology it became possible for the first time to truly monitor an entire populace and to store all the data gathered permanently. Public reaction and, in most jurisdictions, resulting legislation, kept this from becoming a ubiquitous practice until the Technocalypse. In the wake of those events constant surveillance was necessary to survive in the space colonies, where a tiny initial incursion of rogue software, biotech, or nanotech outbreak could spell disaster. Even after the crises passed, the fear remained and no one ever really disbanded the constant surveillance, which humanity carried with it throughout the First Federation Era and eventually to the stars.
With the coming of transapient AIs, it became possible to correlate all that stored data and to analyze it for trends, and cliology was born. Whereas history derives social trends and events from primary source documents and such things as diaries, cliology has a record of every single interaction that ever took place and can therefore make incredibly detailed reconstructions of events based on both data and metadata. The records do not always exist — many primitive worlds lost the background technology, many particular communities as well as many widespread groups such as the Universal Church disapprove of it, and it is not possible to surveil the higher order AIs. All the same, cliology has transformed how people view their pasts.
An important tool in cliology is long-distance observation by advanced telescope, or cliotelescopy as it is known. This allows past events in a distant system to be observed when the light from that system reaches a suitable telescope array.