Steven Pinker

Without the common knowledge, they can maintain the fiction of a platonic relationship.

But if Harry were to have said, want to come up and have sex, and Sally said no, well, now Harry knows that Sally knows that Harry knows that Sally knows.

With this common knowledge, they can no longer maintain the fiction of a platonic friendship.

And that's what lies behind the intuition that with barefaced speech, you can't take it back.

It's out there.

The logic of common knowledge and social relationships explains why we all deplore the hypocrisy and white lies and role-playing of everyday social interaction, but we wouldn't want to live without them, as dystopian comedies like Liar Liar play out.

Our relationships are underpinned by a common understanding of limitless loyalty and generosity.

Privately, we know they're fictions, but making these rude realities common knowledge would pollute the pool of common assumptions that allow us to get along.

We also have to get along in less intimate relationships, and in those cases, we rely on norms, ways of living together that aren't written down by lawgivers or enforced by the police, but exist because everyone knows they exist.

In social life, basic civility depends on norms such as you don't brazenly lie, you don't insult people to their faces, you don't prosecute personal vendettas.

In the anarchic global arena, peace depends on norms such as that nation-states are immortal, national borders are grandfathered in, conquest is unacceptable and nuclear weapons are unthinkable.

These norms are propped up by nothing but common acceptance, and so they're vulnerable to imploding if they are overtly flouted, or even if there is loose talk about flouting them.

I hope that having explained the logic behind human harmony, I don't have to explain how these norms are currently under threat or what is at stake if we lose them.

As a man once said, what's there to say is so obvious.

Thank you.