Here's why:
* Satan's Internal Struggle: Book 3 focuses on Satan's descent into despair and his struggle to reconcile his fallen state with his pride and ambition. He grapples with the consequences of his rebellion against God, facing the loss of Heaven and the reality of his own sinfulness. This internal conflict between his desires and his circumstances embodies the struggle between good and evil.
* The Debate on Free Will: While the conflict is within Satan, it also extends to the debate on free will and the nature of God's justice. Satan uses his own fall to question God's absolute power and the fairness of a system where free will leads to such dire consequences. This raises the philosophical question of how much freedom should be allowed and how much control is necessary to ensure good.
* The Presence of God: Though not physically present, God's omnipresence looms over the entire book. Satan's contemplation of God's power and justice intensifies the conflict between good and evil.
While the conflict is mainly internal, it also has external ramifications:
* Satan's Decision: Satan's decision to tempt Adam and Eve in the Garden of Eden is a direct consequence of his internal struggle. His desire to inflict pain on God and the desire for revenge contribute to his ultimate act of evil.
In essence, Book 3 of Paradise Lost establishes the internal conflict within Satan as the primary setting for the clash between good and evil. It's within his own mind that the battle for his soul and the subsequent consequences for humanity are waged.