So, Uther Pendragon declares "None shall wield Excalibur but me!" and embeds the sword in the stone, where it remains despite the best efforts of the mightiest of Knights. How can this be? Well, Uther put a curse on it. He called upon the gods. "Hey gods, I've been good to you. I want this thing to happen." That's what a curse is. The gods said "mmmmm sounds reasonable, OK. We will keep a grip on that sword for you." I can get behind this. The only required premise of the story is that gods exist. From that everything else works tickety-boo. It is simple to picture some god devoting a molecule of his will to reach down from heaven and place an invisible finger on the sword, such that it cannot moved by man. Now, Thor. Odin declares "Only he who is worthy shall wield Mjölnir!" and embeds the hammer in the ground, where it remains despite the mightiest efforts of humans and fallen gods. Who enforces it? What higher power does Odin, the King of Gods call upon to grant him his request?