Page "Great Race of Yith" Paragraph 4

Although captive minds were prisoners, they were nonetheless granted some freedoms in exchange for their cooperation.

Those captive minds who cooperated with the Great Race were allowed to wander the Yithian cities at will and to browse the Yithians ' gigantic library, which contained metallic cases with books, made of an incredibly long-lasting material, that recorded the histories of uncounted alien races, including humanity.

Creatures inside a Yithian body could also communicate with other captive minds from across our universe ( and beyond ) from the past and future.

Once the Great Race had learned all they could from a captive mind, the occupied being's intellect was swapped back, with the additional precaution of erasing or suppressing all knowledge of the Great Race.

Even so, it is possible for scraps of knowledge or experiences gained from their time with the Great Race to remain in dreams.