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

It's involved in deceit and deception, in politeness, in humoring, in persuading.

We are constantly trying to get inside each other's heads, and this takes it to another

level of getting inside someone's head who's trying to get inside our head, such as when

something happens and I see it, and you see it, and I see you seeing it it and you see me seeing it, that kind of generates

common knowledge in one fell swoop. That's Stephen Pinker, psychologist and bestselling author

whose books like the Better Angels of Our Nature and Enlightenment Now offer mostly optimistic

insights into how evolution has shaped us as social beings. His new book has

the tantalizing title, When Everyone Knows That Everyone Knows, which is a way of describing what Stephen

calls common knowledge. 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 that you know that I know it.

At infinitum. Now that sounds impossible. Your head starts to explode with, you know, two levels

of I know that she knows of I know that she knows,

and I know that he knows, let alone an infinite number.

My head is already exploded.

Well, yes, and it's the subject of humor.

There's a famous episode of Friends in which the, the castor kind of outsmarting each other,

but each one kind of cottons on to the other one's schemes, but, and then they know that they, uh, have been discovered, but they play along and humor each other. And at one point, Rachel says, uh, to Joey, uh, Joey, they, uh, they, uh, they don't know we know, they know, we know. Uh, you can't say anything. He says, I couldn't even if I wanted to.

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