Title:  The Epistemic Logic of Friendship

Abstract: 

Berlin, 1978. A spy network has recently been uncovered by the Stasi, who are rounding up the spies and their associates.Bella is friends with Charlie and Erik, neither of whom are friends with each other. Unknown to the others is that Erik is a spy. The others are not spies, and Erik knows that because all spies know who else is a spy (we suppose). Bella knows that Charlie is not a spy, but Charlie does not know about her. After the network is exposed, all the spies and their friends will be interrogated by the police. But just before this happens a message is relayed to all agents revealing whether or not they are in danger, that is, whether they are a spy (which they would know in any case) or a friend of a spy. Who now knows that Erik is a spy?

Manhattan, c. 1950.  Peggy knows that Roger is cheating on his wife, Mona. What’s more, Roger knows that Peggy knows, because they met accidentally while he was with his mistress. Mona does not know about the affair, and both Peggy and Roger know this. The situation (for Roger) deteriorates when he discovers that Peggy is a terrible gossip. She is bound to have told all her friends about his affair. What Roger does not know is whether Mona is a friend of Peggy (she is). Who knows what, exactly?

I will present a logic for reasoning about such social situations and use it introduce some distinctions between different kinds of social knowledge. This is joint work with Patrick Girard (Auckland) and Liu Fenrong (Tsinghua).

主讲人：Jeremy Seligman

主持人：郭佳宏

时间：6月13日，周四，下午3：00-5:00

地点：北师大教四楼106