SpaceX founder and Tesla CEO Elon Musk is a bookish billionaire — as we've reported before, he has a love of all things science fiction.

He's actually tweeted about a favorite book that's helped clarify his galactic ambitions.

"Reread Asimov's Foundation series," he said. "Brilliant."

In interviews, Musk has reiterated that statement, saying that "Foundation" by Isaac Asimov is "one of the best books" that he's ever read. He loved it so much that SpaceX's Falcon Heavy rocket, which launched in January 2018, included a copy of each of the books in the trilogy.

The books tells the story of an intergalactic empire that falls to pieces, with a dark age waiting on the other side.

"Foundation" is a "futuristic version of Gibbon's 'Decline and Fall of the Roman Empire,'" he told the Guardian, referencing the famous history that followed Rome from its breathtaking heights to the fall of Byzantium.

From "Foundation," Musk learned that every civilization — including our own — will one day falter.

"The lessons of history would suggest that civilizations move in cycles," Musk told the Guardian. "You can track that back quite far — the Babylonians, the Sumerians, followed by the Egyptians, the Romans, China. We're obviously in a very upward cycle right now, and hopefully that remains the case. But it may not. There could be some series of events that cause that technology level to decline."

And when that happens in our case — when humanity hits its own dark age — what's the best thing we can do to make sure it's as short as possible?

By having the human race become, to use Musk's favored terminology, interplanetary.

Thus the need for SpaceX, the privatized space exploration company that Musk started back when only nation-states were trying to launch rockets out of Earth's atmosphere.

"Given that this is the first time in 4.5 billion years where it's been possible for humanity to extend life beyond Earth," he said, "it seems like we'd be wise to act while the window was open and not count on the fact it will be open a long time."