It can be tough to get people to truly care about reducing existential risks today. But spare a thought for the longtermist of the 17th century: they were surrounded by people who thought extinction was literally impossible.

 Today’s guest Tom Moynihan, intellectual historian and author of the book X-Risk: How Humanity Discovered Its Own Extinction, says that until the 18th century, almost everyone — including early atheists — couldn’t imagine that humanity or life could simply disappear because of an act of nature.

Links to learn more, summary and full transcript.

This is largely because of the prevalence of the ‘principle of plenitude’, which Tom defines as saying:

“Whatever can happen will happen. In its stronger form it says whatever can happen will happen reliably and recurrently. And in its strongest form it says that all that can happen is happening right now. And that's the way things will be forever.”

This has the implication that if humanity ever disappeared for some reason, then it would have to reappear. So why would you ever worry about extinction?

Here are 4 more commonly held beliefs from generations past that Tom shares in the interview:

• All regions of matter that can be populated will be populated: In other words, there are aliens on every planet, because it would be a massive waste of real estate if all of them were just inorganic masses, where nothing interesting was going on. This also led to the idea that if you dug deep into the Earth, you’d potentially find thriving societies.
• Aliens were human-like, and shared the same values as us: they would have the same moral beliefs, and the same aesthetic beliefs. The idea that aliens might be very different from us only arrived in the 20th century.
• Fossils were rocks that had gotten a bit too big for their britches and were trying to act like animals: they couldn’t actually move, so becoming an imprint of an animal was the next best thing.
• All future generations were contained in miniature form, Russian-doll style, in the sperm of the first man: preformation was the idea that within the ovule or the sperm of an animal is contained its offspring in miniature form, and the French philosopher Malebranche said, well, if one is contained in the other one, then surely that goes on forever.

And here are another three that weren’t held widely, but were proposed by scholars and taken seriously:

• Life preceded the existence of rocks: Living things, like clams and mollusks, came first, and they extruded the earth.
• No idea can be wrong: Nothing we can say about the world is wrong in a strong sense, because at some point in the future or the past, it has been true.
• Maybe we were living before the Trojan War: Aristotle said that we might actually be living before Troy, because it — like every other event — will repeat at some future date. And he said that actually, the set of possibilities might be so narrow that it might be safer to say that we actually live before Troy.

But Tom tries to be magnanimous when faced with these incredibly misguided worldviews.

In this nearly four-hour long interview, Tom and Rob cover all of these ideas, as well as:

• How we know people really believed such things
• How we moved on from these theories
• How future intellectual historians might view our beliefs today
• The distinction between ‘apocalypse’ and ‘extinction’
• Utopias and dystopias
• Big ideas that haven’t flowed through into all relevant fields yet
• Intellectual history as a possible high-impact career
• And much more

Chapters:

• Rob’s intro (00:00:00)
• The interview begins (00:01:45)
• Principle of Plenitude (00:04:02)
• How do we know they really believed this? (00:13:20)
• Religious conceptions of time (00:24:01)
• How to react to wacky old ideas (00:29:18)
• The Copernican revolution (00:36:55)
• Fossils (00:42:30)
• How we got past these theories (00:51:19)
• Intellectual history (01:01:45)
• Future historians looking back to today (01:13:11)
• Could plenitude actually be true? (01:27:38)
• What is vs. what ought to be (01:36:43)
• Apocalypse vs. extinction (01:45:56)
• The history of probability (02:00:52)
• Utopias and dystopias (02:12:11)
• How Tom has changed his mind since writing the book (02:28:58)
• Are we making progress? (02:35:00)
• Big ideas that haven’t flowed through to all relevant fields yet (02:52:07)
• Failed predictions (02:59:01)
• Intellectual history as high-impact career (03:06:56)
• Communicating progress (03:15:07)
• What careers in history actually look like (03:23:03)
• Tom’s next major project (03:43:06)
• One of the funniest things past generations believed (03:51:50)

Producer: Keiran Harris.
Audio mastering: Ben Cordell.
Transcriptions: Sofia Davis-Fogel.