Hello everyone and welcome to the Mindscape Podcast. I'm your host Sean Carroll.

And you know if you're a long time listener that I tend to mix things up from week to

week, the episodes are not really related to each other for the most part. But this week's

episode can in some sense be thought of as a philosophical companion piece to last week's

episode. Last week we had Max Teigmark talking about the mathematical multiverse. Today

we have Ned Hall who's a philosopher in fact chair of the philosophy department at Harvard

University who's done absolutely important work on causality on the laws of nature and

also on the idea of possible worlds. The philosopher David Lewis who was very well known in philosophy

circles, although less well known to the general public, examined this idea of all possible

worlds and how they could be related to each other, not physically, not sort of causally,

but the different ways in which they differed from each other and how those ways entered

into our analysis of counterfactual conditions, examining possibilities that didn't actually

come true in our universe. So I talk with Ned about causation and the laws of physics,

as I said, and how he analyzed these ideas in the context of imagining all the different

possible universes out there. Now Lewis thought that all the different possible universes

were actually real, that they actually existed. That's very close to Max Teigmark's idea

of a mathematical universe where every mathematical structure actually exists. But it's

a little bit different. I think you can actually use the idea of possible worlds in a way

that doesn't necessitate that you believe that they're all real. But whatever your position

on that is, we're going to get there today. This is a wonderful conversation. Ned is just

a true genius at taking esoteric philosophical ideas and bringing them down to earth and making

them very understandable. So let's go.

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