Monday, February 11, 2008

Blue-eyed Islander Puzzle - an analysis

Many people find themselves stumped by the so-called Blue-Eyed Islanders puzzle. There is also much controversy over its supposed solution. I'm going to analyze the problem and the solution, and in the process, explain why the solution works.

To begin, let's modify the problem slightly and say that there's only 1 blue-eyed islander. When the foreigner makes his pronouncement, the blue-eyed islander looks around and sees no other blue eyes, and being logical, correctly deduces that his own eyes must be blue in order for the foreigner's statement to make sense. The lone blue-eyed islander thus commits suicide the following day at noon.

Now comes the tricky part, and the source of much confusion. Let's say there are 2 blue-eyed islanders, Mort and Bob. When the foreigner makes his pronouncement, Mort and Bob look around and see only each other. Mort and Bob thus both temporarily assume that the other will commit suicide the following day at noon. Imagine their chagrin when they gather in the village square at noon the next day, and Mort and Bob both look at each other in surprise because Mort thought Bob was going to commit suicide, and Bob thought the same of Mort! Now both Mort AND Bob know their own eye colour is blue, and they will both commit suicide on day 2.

The very same argument can be extended to 3 blue-eyed islanders, Mort, Bob and Sue, who will commit suicide on the third day at noon. The day of the pronouncement, the three of them see each other, and Sue assumes Mort and Bob see only each other. Being logical, she thus deduces that they will commit suicide on the second day, by the above argument. Mort and Bob each see the same number of blue eyes as Sue, and thus reach the very same conclusions.

Imagine Sue's chagrin, when she gathers in the village square on the second day, and Mort and Bob are both surprised that she's not committing suicide! She now knows that her eye colour is also blue, and all three of them will kill themselves on the third day.

This inductive argument can be generalized to N blue eyed islanders, where all N of them will suicide on the Nth day after the pronouncement. QED.