Paradise Lost takes place almost exclusively in a time and place when death, sin, and lying didn't exist. In other words, it deals with a time when humanity was still innocent. While Milton is very good at painting a picture of innocence, he realizes and reflects on the difficulty of attempting to describe something as alien as a deathless, sinless existence.
Questions About Innocence
Is there a moment in your life when you feel you lost your innocence?
Would you want your children to never lose their innocence?
What would a perfect world look like?
What other works of literature (or movies) deal with the loss of innocence?
Chew on This
Just because Adam and Eve could have kept Paradise forever does not mean that God wanted to keep them totally innocent; Raphael makes it clear that there would have come a time when they achieved some degree of experience.
Milton's attempt to describe innocence ultimately fails, if only because he must present it through the fallen medium of human language.