If textbooks allowed for controversy, they could show students which claims rest on strong evidence, which on softer ground. As they challenged students to make their own decisions as to what probably happened, they would also be introducing students to the various methods and forms of evidence—oral history, written records, cultural similarities, linguistic changes, human genetics, pottery, archaeological dating, plant migrations—that researchers use to derive knowledge about the distant past. Unfortunately, textbooks seem locked into a rhetoric of certainty.
Source: Loewen, James W.. Lies My Teacher Told Me: Everything Your American History Textbook Got Wrong (p. 41). The New Press. Kindle Edition.