Jon Pretty

100 brown-eyed and 100 blue-eyed perfect logicians live on an island, but none knows the color of their own eyes. There are no mirrors on the island, and communicating is forbidden. If any of them works out the color of their eyes, they must leave the island in the night.

One day a visitor comes to the island and says, "I see at least one person with blue eyes". What happens next? The surprising result is that nothing happens for a hundred days, then all the blue-eyed people leave, and the next night all the brown-eyed people leave.

1) I've heard this phrased differently, where a person leaves only if they have blue eyes and know this. Changes the outcome slightly (everyone else doesn't leave the next day), but not the reasoning approach.. 2) Your outcome is wrong. Consider the case where there's 1/

... exactly 1 person w/ blue eyes. They don't wait until day 100 to leave, right? They leave the first night! (and then in your version, everyone else leaves the next night). 3) The problem itself is actually less interesting to me than this followup question. Pretend for 2/

... the moment that there are at least N>=3 people with blue eyes on the island. Since everyone can see everyone else by assumption, then everyone ALREADY knows that there's a person with blue eyes, even before the visitor announced it! So, what purpose does the visitor 3/