There are several reasons that Paradise Lost is considered in Epic. First, in the formal definition of the term, an epic is a long narrative poem utilizing elevated style. Paradise Lost fits this definition because of its content, length, and style (the use of figurative language, inverted sentences, caesura, etc). Another trait of epics is a vast setting, covering a long period of time. The setting of Paradise Lost is the entire Universe, including Heaven and Hell, which definitely qualifies as vast. Traditional, epics invoked the Muses, or some other supernatural force, when presenting a story. Milton does this as well, referring to "Heavenly Muse" at the opening of the story. Paradise Lost also involves external conflict, centering on the battle between good and evil (God versus Satan). This is another trait typical of an epic. All of these conventions allow Paradise Lost to be classified in this category.