When you look at Milton's Paradise Lost as an epic, according to the traditional interpretation of what constitutes an epic, you will find several parallels to classic epics like The Odyssey or Gilgamesh, despite the difference in subject matter.
First: an epic is generally in the form of a long poem; it can rhyme or not, but it does generally have consistent meter. Paradise Lost does conform to the traditional format.
Second: an epic is generally historical and broad in scope, having to do with the building of a nation or the formation of a society, involving the deeds of heroes. Again, Paradise Lost fits the paradigm, but with a twist. The hero is Lucifer, the fallen angel, and the nation building is actually the story of Lucifer attempting to destroy heaven and replace it with his own idea of the world.
Third: the epic is generally a moral tale, and, though unconventional, Paradise Lost is a moral tale as well, demonstrating Milton's own moral code and his ideas of what happens when one tries to go against God. Even though Lucifer is portrayed as a sympathetic character, God wins in the end.
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