How did Plotinus explain the existence of evil?

For Plotinus, the existence of evil is not an independent principle or a dualistic force opposing the Good. Instead, he defined evil as a lack or absence of being.12

In his philosophy, the universe expands from the One—the supreme, perfect source—through diminishing degrees of reality. At the absolute limit of this expansion lies matter, which represents the final absence of being. Because matter is characterized by this utter negativity, Plotinus identified it as the principle of evil.1

Furthermore, Plotinus viewed the universe as a Great Chain of Being characterized by plenitude and gradation. He argued that the goodness of the universe as a whole depends on its fullness, which requires the existence of all possible varieties of beings. Consequently, because all beings other than the perfect source are inherently imperfect, the best possible world must necessarily contain all possible degrees of imperfection and evil. Within this cosmic harmony, individual suffering is seen as a necessary element of the greater pattern rather than a disruption of the world's overall beauty.21