Premise 3 in Leibniz's cosmological argument, known as the "Principle of the Best Possible World," might indeed appear unnecessary at first glance, as it introduces an additional layer to the argument. However, it serves a specific purpose in Leibniz's overall philosophical system and in his attempt to address certain theological and metaphysical questions. Here are a few reasons why Leibniz included this premise:
Addressing the Problem of Evil: Leibniz was well aware of the problem of evil, which questions how an all-powerful, all-good God can coexist with the existence of suffering and imperfection in the world. By positing that God creates the "best possible world," Leibniz attempts to provide a theodicy, a defense of God's goodness and omnipotence in the face of evil. He argues that while the world may contain imperfections and suffering, it is the best possible world that can exist given the constraints of the other premises.
Emphasizing Divine Perfection: Leibniz's philosophy often focused on the idea of God as a perfect being. By including the third premise, he underscores the notion that a perfect and benevolent God would create a world that reflects the highest degree of goodness, order, and harmony. This aspect of his argument aligns with his broader theological and philosophical views.
Highlighting God's Role as Creator: The third premise emphasizes God's active role as the Creator of the universe. It suggests that God didn't just set the world in motion and then step back; rather, God continually sustains and governs the world to ensure that it remains the best possible world. This active involvement of God in the ongoing existence of the world is significant in Leibniz's metaphysical system.
Providing a Comprehensive Explanation: Including the third premise helps Leibniz offer a more comprehensive and satisfying explanation for the existence of the contingent world. It goes beyond merely asserting the existence of a necessary being as the ultimate cause and offers a glimpse into the nature of this necessary being (i.e., a perfect God).
While premise 3 might not be strictly necessary for a cosmological argument in its basic form, it is an integral part of Leibniz's unique version of the argument, which seeks to reconcile the existence of God with the presence of both contingent beings and the problem of evil. It reflects his broader philosophical and theological concerns, making his cosmological argument a distinctive contribution to the history of philosophy.