Ammon says he knew of a man who could "look and translate all records that are of ancient date . . . and the same is called a seer" (Mosiah 8:13). According to Brant Gardner, the key to understanding what Ammon is saying here is the word "ancient." That is, we are dealing with "ancient" records. To understand the importance of the ancient text, we need to understand the way the past was conceived in ancient Israel and in ancient Mesoamerica (and likely in many other parts of the ancient world). History was not simply something that had happened, but a glimpse at cycles that had occurred and will occur again.
For Israel, "the unifying principle [acted] like a magnet in evoking a pattern amongst iron filings. It created a pattern of history out of all its complexities, a pattern which disclosed the previously hidden purpose of God." Thus for Israel, the past revealed the form of the future. One of the manifestations of this patterning of life and history can be seen in the numerous ways in which the Exodus became the model for subsequent events, including Lehi and his family.
In Mesoamerica, all time ran in repeating cycles. The creation myth that was shared among the Maya and Nahua told of recurring cycles of destruction and new creations in which the destruction/renewal of the sun was the principle event. The serendipitous arrival of the Spanish in a Mesoamerican cyclical year which symbolized change and renewal allowed them to be seen as a predicted return of the god, Quetzalcoatl. Sadly, however, the greatest repetition of the past was not Cortez' arrival becoming the triumphal return of Quetzalcoatl, but the eerie way that the destruction of the Aztec kingdom repeated the destruction of ancient Tula--an event also linked indelibly to Quetzalcoatl in Nahua mythology.
A seer, therefore, was one who could not simply read about the past, but have the past revealed to him--to have the "real truth" of the past revealed, not simply the record of the past. With a conception of the past that linked it to the future, a seer was then one who could see the future because he could see the past, whereas a prophet saw only the future that was revealed to him. As a seer would have the larger patterns available to his prophecy.
Notice how king Limhi immediately understands this concept by his response to Ammon: "a seer is greater than a prophet" (Mosiah 8:15). It is then that Ammon amplifies what the king has said. [Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/~nahualli/LDStopics/Mosiah/Mosiah8.htm], pp. 14-15]