How does Milton manipulate debate as a form in "L'Allegro" and "Il Penseroso"?
Though Milton's speakers set their arguments against each other in their introductions, the poems that follow are closer than they initially appear. Milton blurs the distinction between his speakers' arguments, subverting their debate by drawing attention to what their poems share. Though the speakers claim that their lives are totally opposed, Milton makes it clear that they're living in the same world. He presents a debate in order to knock it down.
Why is "Il Penseroso" often thought of as a night poem?
Many consider "Il Penseroso" to be Milton's night poem because so many of the scenes are set at night. It's true that much of the poem takes place in the dark, but the poem is also full of day scenes. Similarly, "L'Allegro," which many consider to be Milton's day poem, includes both day and night scenes. Like many of the binaries Milton sets up in "L'Allegro" and "Il Penseroso," the day/night division doesn't hold. What Milton is doing in both poems is perhaps better represented by the scenes he sets in mixed light, like the lines in which his speaker lies in the shade during the day in "Il Penseroso." Milton's poems include light and dark, because more than he's interested in representing division, he's interested in exploring what his speakers share.