Steven Pinker

Rose means rose because everyone knows it means rose, and everyone knows that everyone knows.

So all of our conventions, all of our ways of coordinating, all of our harmony depends on common knowledge.

Just to give a concrete example from Thomas Schelling, the political scientist and economist who

was one of the originators of the concept.

Imagine that, say, a husband and wife get lost in Manhattan.

This is the era before cell phones.

How can they meet up?

He can think, well, she likes to go to a bookstore, so I'll meet her there.

But then he thinks, oh, but she knows that I like to go to a camera store, so maybe she'll go to the camera store.

But then she knows that I know that she likes to go to the bookstore.

She'll go to the bookstore after all.

Meanwhile, each of them can kind of ricochet with this useless empathy and still not end up at the same place at the same time.

Nothing short of common knowledge, not only knowing something, but knowing that the other person knows that you know, gets them together.

Now, common knowledge can be generated by...

That's how I got into it.

That is, in this case, a cell phone call.

Although it can also just be a convention, something that everyone assumes that just coordinates.

Like, what day of the week do you stay home?

Sunday.

Why Sunday?