Here, the knowledge used by pursuers is a law of excluded middle. The pursuer sees that a sentence written on the lead box and a sentence written on the gold box are mutually negative, which is two contradictory judgments. According to law of excluded middle, there must be a truth in two contradictory judgments. According to the meaning of the question, only one sentence is true, and this truth can only be judged in these two contradictions, so the sentence on the silver box is a lie. Since the words "the portrait is not in this box" on the silver box are false, it can be concluded that the portrait is in the silver box.

Give two more examples of law of excluded middle.

When Trotsky joined the Bolshevik Party, he issued a statement: "I joined the Bolshevik Party ... This fact itself has proved that I have put everything that separated me from Bolshevism in the past at the door of the Party." Trotsky's statement is intended to confuse the audience with an image to cover up his political speculation. You can keep your differences or discard them, that is, you can accept them or discard them. There is no third option. But Trotsky cunningly said "put it at the party gate" and denied that it was inside or outside the door. Therefore, Stalin revealed in a tit-for-tat manner: did he store these things at the door of the party for future inner-party struggles, or did he simply burn them?

Mr. Lu Xun once wrote a short essay "Debate", which vividly exposed the ugly state of the "fence-sitter" in either case. The record is as follows:

I dreamed that I was preparing my composition in the lecture hall of a primary school, and asked the teacher how to do it.

"difficult." The teacher squinted at me from his glasses and said, "I'll tell you something."

"A family gave birth to a boy, and the family was very happy. At the full moon, show it to the guests-probably naturally, I want to get a good sign.

"One said,' This child will be rich in the future.' So he got some thanks.

"One said,' This child will be an official in the future.' Then he took back a few words of praise.

"One said,' This child will die in the future.' So he was jointly beaten.

"Said the inevitable death, said rich lies. But liars are rewarded, and they will fight if they say so. You ... "

"I am willing to lie and be beaten. So, teacher, what can I say? "

"Then you say,' Oh, this child. Look, how could ... ouch. Ha ha. Hehe. He, hehehe' ah. "