Past history and tales, practices and caveats passed from generation to generation via vocalizations and repeated listening. Since then the thought of being literate is really the idea of possessing specific knowledge. Neither functions as well without the others as compliment and balance. All are symbols of thought. Prior knowledge is the cornerstone upon which present and future knowledge adheres, it is the buttress of new ideas and the anchor of even the most radical suppositions.
Prior knowledge is the bottomless well of writing ideas and the warp drive of reading comprehension.
Writers and readers use an array of cueing systems as they make meaning. The first may be graphic cues, pictures which correspond with the text or simply the uniqueness of the word itself in graphical terms, use of grapho-phonemic information, understanding trends in letters and sounds. Syntactical or structural cues, semantic, typological, and morphological round out the cueing systems of which I am presently aware. Strategies involved in reading and writing vary but some strategies seem to remain paramount. The prediction cycle begins with schemas somewhere between sparse and elaborated.