The surprise tiger

* Always the unexpected happens ... 

The Princess and Louis loved each other tenderly . But the king did not approve . So he launched a challenge to Louis :

    'You 'll have to kill the tiger which is hidden in locked rooms 5 by 5 grids. But it will be a surprise for you can no tiger guess where is the tiger.

Then Louis watched the gates 5 and said: -if the first four gates open four empty rooms , I know that the tiger is behind the fifth . But as the king has said that I can not guess where is the tiger, it can not be behind the grill No. 5 . -the # 5 thus eliminated , tiger must be in one of four parts. What happens if I open three empty rooms ? The tiger will surely find the room No. 4, but it will not be a surprise since it can be either in the room 's eliminate No. 5. So the grid No. 4. By the same reasoning, Louis concluded that the tiger may be behind other grids. It is reassured and he exults : no tiger is behind the bars , in fact if there was one it would not be unexpected. Also here are the 5 grids below , open each one by clicking on it with Louis .

Hopefully the tiger was averted in time! ; o)

Surprising as it may seem, there is indeed a tiger behind a grille . The king was right , you were surprised whatever the grid behind which was the tiger ! If you arrived before the fifth door, you thought there was no tiger since it would necessarily be in that cage and thus the king was mistaken . But the tiger was there and suddenly you have been very surprised !

 

This paradox exists in other forms : 

hanging surprise that a prison director would be visited one of his prisoners ... 

or surprise inspection : a teacher told his students that there will be a surprise inspection in the week : Monday to Friday. Students argue thus : if the teacher chooses to do surprise check on Friday , so Thursday night students will know that the inspection will take place the next day and it will not be a surprise. So control will not take place on Friday . And so on ... as in the problem of the tiger , students conclude that the teacher can give surprise control. Yet if the teacher chooses Wednesday , for example, how could we know that monitoring will take place that day ?

Logicians are puzzled by this paradox 

If the king knows that he can keep his word , Louis know, so it has no reason to consider as valid a deduction found no tiger behind the bars including the last . In the issue of control , if the teacher chooses Wednesday , how will they know the students they will have a interro that day? Ian Stewart lost in paradoxes and Found For Science in August 2000 think the paradox is based on the ambiguous use of the word "surprise" . If every moment we expect a surprise, the surprise is no longer a ... Even if students say the control date is not a surprise they will have to work and revise every night to control! And conclude that this paradox is a paradox lost.