Practitioners seek to revive these past belief systems by using surviving historical source materials. Among the historical sources used are Old Norse texts associated with Iceland such as the "Prose Edda" and "Poetic Edda", Old English texts such as "Beowulf", and Middle High German texts such as the "Nibelungenlied". Some Heathens also adopt ideas from the archaeological evidence of pre-Christian Northern Europe and folklore from later periods in European history. Among many Heathens, this material is referred to as the "Lore" and studying it is an important part of their religion. Some textual sources nevertheless remain problematic as a means of "reconstructing" pre-Christian belief systems, because they were written by Christians and only discuss pre-Christian religion in a fragmentary and biased manner. The anthropologist Jenny Blain characterises Heathenry as "a religion constructed from partial material", while the religious studies scholar Michael Strmiska describes its beliefs as being "riddled with uncertainty and historical confusion", thereby characterising it as a postmodern movement.