Well, to be even more pedantic, there is a rule for leaving out the final s. You leave it out in possessives built from names of famous historical personalities

Well, to be even more pedantic, there is a rule for leaving out the final s. You leave it out in possessives built from names of famous historical personalities and mythical creatures: Socrates' dialogues, Zeus' wife, Pegasus' wings, Jesus' teachings. In every other case you have to add the s: Bayes's Theorem, Jones's wife. At least that's how I've learned it as a non-native speaker.