I'm Rufus Griscombe, and this is the next big idea.

Today, lessons in common knowledge and rationality. The legendary Harvard psychologist Stephen Pinker published a new book this week.

It's called When Everyone Knows That Everyone Knows.

And it's about, well, I'll let Steve tell you.

My latest book is about how people think about what other people think they think.

I know.

It sounds like a riddle.

Really what the book is about is this process by which we go from private knowledge.

I know something and you know it.

To common knowledge.

I know that fact and you know it, but in addition, I know that you know it and you

know that I know it.

On top of that, I know that you know that I know that I know it.

And you know that I know that you know it.

And so on adding the item.

Steve says this shift is the hidden force behind everything from financial bubbles to political protests to why we say Netflix and chill instead of just asking for sex.

Common knowledge doesn't just help us coordinate.

It shapes our economy, our politics, and our most intimate relationships.

In his previous books, The Better Angels of Our Nature and Enlightenment Now,

Steve applied a contrarian lens to human history, revealing that life on the whole is getting better.

Now he's applying that same contrarian lens to our social interactions, revealing the

invisible operating system that makes human coordination possible.

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