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III 2: On Providence (1)
The world is the product of both Reason and Necessity. Being a divine creation and perfect, the world is beautiful as a whole: though the parts of it that are of Necessity may be ugly. But is it fair to denigrate the whole because of a part? Is blessed Achilles to be condemned because of Thersites, "the ugliest man who came to Troy?"
Life in this world may be likened to a play. The World Soul is the playwright, individual souls are actors, bodies are the characters those actors play, and the material world is the stage. But the greatest plays require drama, and the harm or even the death of characters. But does this trouble the actors? Of course not—they play their parts to the best of their ability, appreciating that the travails of their character are necessary to the story. And if a character dies, what is it to the actor? They simply go backstage, change their costume and makeup, and play a new character in the next act—just as souls do when they take on new bodies. And neither should we consider the actors passive: they did not write their role, sure, but they have considerable freedom to act and improvise within the bounds of the story, and so the greatest actors are celebrated nearly as highly as the playwright themselves.
If this is not enough to go on, let us consider an example: why do the wicked prosper? Because they do the work, of course! The gods help those who help themselves: do you think that if a man who, depraved of morals, has nonetheless trained his body or mind to be strong should have any trouble despoiling those who have not? Even such a man is not pure wickedness, for the discipline and practice it takes to strengthen himself is good! If you wish to protect yourself from him, follow his example and take matters into your own hands—see, the gymnasium and the academy are there before you!—rather than blame Providence for your feebleness.

For sake of sightseeing, let me call out some sections of particular interest: §3, where the material world pleads its nobility, is moving; §13 has an excellent description of karma; and Plotinus' elegant analogy of the play is from §15.