@watchanddream Sure! This got a bit lengthier than I intended, but I wanted to try and be thorough.
Put far too simply, xenia is the practice of hospitality, but it's also a ritual and in a sense a contract between guest and host. When a stranger arrives at your doorstep, you're expected to feed them and give them a gift as the host. Typically, you're not even supposed to ask them their name until they've finished their meal. Once you've done this as the host, the guest is now expected to share stories and information about themselves, and not burden you or your household. Typically, people focus on how hosts violate xenia, but guests can also violate xenia.
This is called a guest-friendship, and once it's established, you are expected to offer the same to your host should they ever visit. It's a ritual of hospitality, yes, but it's also used to establish alliances and can be passed down through generations. For example, in the Iliad, when Glaucus and Diomedes discover that their grandparents were guest-friends, they decide not to fight, but rather exchange armor.
Theoxeny is a common story you see in myth where a god (typically Zeus) disguises themselves as a mortal and requests to be hosted by someone. Should the host follow the rules of xenia, they'll be rewarded, but if they fail, they'll be punished or killed.
This is important to the Odyssey, as it comes up multiple times, but in particular, the cyclops, and the suitors. In both these instances, Odysseus arrives disguised and gives a false name, the practices of xenia are failed, and Odysseus reveals himself and punishes the one that failed to follow xenia. The suitors in particular fail xenia, as they both fail as hosts, AND as guests, for burdening Odysseus's household.
Xenia being treated as the golden rule comes across to me like a Christian reinterpretation, as while many Greek philosophers did have something similar to the golden rule, those were philosophers, and the idea that it's a religious command from a god seems much more in line with a Christian idea of where the rule comes from. (Note, I'm aware that other religions had something similar, even as a command from a god, but I highly doubt Nolan is drawing inspiration from those instances)
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