I'm Alan Alda, and this is clear and vivid conversations about connecting and communicating.

Welcome to a preview of our next season, season 31, which starts a week from now. We've got some wonderful guests

lined up for you. I think you'll be fascinated with them. I was. And I'm here with our executive

producer, Graham Ched, who just discovered something that links our lineup of guests,

something we weren't aware of when we were inviting them to be on the show.

When we go about deciding who to invite us guests, we aren't usually looking for connections

between them, even though our theme is connecting and communicating.

But the five guests we're going to be giving you glimpses of now turn out to be connected

in unexpected ways.

First up is famous psychologist and best-selling author Stephen Pinker,

whose new book has the slightly mysterious title,

when everyone knows that everyone knows dot, dot, dot.

They trip me up a little bit because he goes out to infinity.

When I know that you know, that I know. When I know that you know that I know that you know that I know,

but it all makes sense in the end.

Stephen, I really couldn't wait to talk with you about this.

You have a concept here that seems to apply to everything we do with one another as humans.

Tell me what common knowledge is.

Yes, so common knowledge in the technical sense.

It refers to the state where I know something and you know it and you know that I know it

and I know that you know it and you know that I know that you know it. And you know that I know that you know that I know it and I know that you know it and you know that I know

that you know that you know that you know that you know that you know that I know it at infinitum

...