WARNING: CONTAINS SPOILERS.
Star Wars is undoubtedly one of the most magical and beloved sci-fi/fantasy film series of all time. Episode IV gave us an incredibly memorable cast of heroes and villains, so much so that, about thirty years later, the masterminds behind Rogue One: A Star Wars Story could not resist using a bit of CGI to reconstruct two of them. From the infinite optimism of Luke Skywalker and Princess Leia to the rugged individualism of Han Solo, Star Wars has captured multiple generations of fans.
At the risk of writing hagiography, I insist that the original Star Wars trilogy (Episodes IV-VI) is pure magic. However, where the prequels (Episodes I-III) lack in stunning character development, I believe they compensate for it in terms of plot and lore building. In the original trilogy, the origins of the sorcerer Darth Vader are left largely mysterious. It is simply known that he was once a Jedi Knight. But just what are the alternatives to the way of the Jedi? In terms of the movies themselves, it is not until the first prequel (Episode I: The Phantom Menace) that answers to this question are provided.

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WARNING: THE FOLLOWING CONTENT CONTAINS SPOILERS.

To learn about the material in a more “proper” fashion, see Star Wars in the order in which the films were released (IV, V, VI, I, II, III).

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According to Episode I, the dark counterparts to the Jedi are called “Sith”. In conversation with Mace Windu, Yoda comments that there are always supposed to be two Sith at a time. Why is this? In this post, I suggest that a plausible answer to this question can be constructed using a conceptual framework that is provided by Christian metaphysics.
By “Christian metaphysics”, I intend to refer to a famous line of thought taken by St Augustine of Hippo in response to the simultaneous existence of God and evil (although there are of course many more topics in the category of metaphysics other than good and evil). The logic is fairly straightforward. God is the creator of all that exists, and yet evil exists. Is God therefore evil? According to the Bishop of Hippo, the answer to the question that is begged is an emphatic no. In the same way that darkness is not a creation of light, but merely the privation of light, so also Augustine reasoned that evil is not a creation of good (God being the wellspring of all that is good), but a privation of it. As God is light without darkness, so whatever darkness there is cannot be a consequence of God’s inviolable light.
One important consequence of Augustine’s thinking is that the good (God) is coincident with “being” (in the classical metaphysical sense). Whatever evil exists only exists as a parasite of goodness. Badness cannot create being; it can only distort it. J. R. R. Tolkien understood this principle well by seeing evil in Middle-Earth as a parasitic force against goodness. The demon Morgoth cannot create goblins; he can only mutilate elves, thereby turning them into goblins. There is a similar relationship between good and evil, according to Augustine. Good is a force of transcendent creativity; evil is fundamentally boring in itself, and whatever attraction it has is only a consequence of the goodness that it twists.
I believe that Christian metaphysical thinking can go a long way in explaining the reason why there are only ever two Sith at a time. Unlike the Jedi, the Sith use “the dark side of the force”. Perhaps, just as evil in Christian metaphysics is a necessarily finite force that is ultimately doomed to perish (Gregory of Nyssa), evil in Star Wars is inferior to goodness. Whereas the light side of the force is a limitless creative energy, akin to the fullness of God’s divine light in our world, the dark side is invariably limited and limiting.
It may be that the Sith’s “rule of two” was invented to take advantage of this. In the Jedi Order, hundreds of Jedi could share the infinite light of the force, and infinity divided by several hundred (so to speak) would still be infinity in each case (although the light side is more challenging than the dark side because of the greater amount of virtue and discipline that is required to commune with goodness). On the other hand, the finite dark side of the force could theoretically be significantly diminished by a large number of Sith trying to tap into its limited potential at once.
I propose that the rule of two was developed to overcome this ontological setback. If there are only two Sith in the universe who share the finite resources of the dark side, then those two people will each still have a tremendous amount of power. Although according to this theory the dark side is not infinite, it is still presumably quite sizeable. Perhaps the entire dark side of the force can at maximum size be 500,000 imaginary units. If there were one thousand Sith, each Sith Lord would only have 0.1% of the total energy of the dark side (500 imaginary units). But if there are only two Sith, each sith would have 50% of the total energy of the dark side (250,000 imaginary units) — five hundred times the amount of imaginary units that each of one thousand Sith Lords would have.
This theory accords with the available evidence. Somehow, mysteriously, the Sith make some highly impressive achievements in the prequels. If they each are drawing from a tremendous amount of power, then their accomplishments make sense. Of course, each Jedi is, according to my theory, still drawing from an infinite potential. Why aren’t the Jedi overwhelmingly more powerful than the Sith, then? That can be explained by the discipline and rigor that is required to use the light side of the force. Just as being virtuous in our world is more difficult than being vicious, so also using an infinite wellspring of light in Star Wars is more difficult than using the immediately available but ultimately inferior power of the dark side.
There are takeaways and lessons for our world to draw from thinking about darkness and light in Star Wars. In our world just as much as in the galaxy of Star Wars, evil is ultimately finite and infinitely banal. It would be below our dignity as creatures called into being by the light of God to neglect Jesus’s call to holiness. We should devote our imagination and our creativity to growing in the grace and in the knowledge of Jesus, who teaches us how to be human in a world that is wounded and craven.