After a shipwreck near a distant island, a group of six survivors gather by the fire in the home of a native who offers them shelter. Each of them pronounces one of the following phrases:
- What weekday is today? I'm sure it's not Tuesday, Wednesday or Thursday.
- Yesterday was Friday.
- No, now that I think about it, yesterday was not Friday, tomorrow is Friday.
- The day after tomorrow is Thursday.
- You are wrong, tomorrow is Thursday.
- I think today is Thursday.
- All we know for sure is that today is not Monday.
Due to disorientation, all shipwrecked people are wrong and only the native makes a correct statement.
What day is today?
In the first statement they tell us that today is Monday, Friday, Saturday or Sunday.
According to the second statement today is Saturday.
The third sentence tells us that today is Thursday.
According to the fourth, today is Tuesday.
The fifth tells us that today is Wednesday.
The sixth person tells us that today is Thursday.
The last person doubts between Tuesday, Wednesday, Thursday, Friday, Saturday or Sunday.
From all the conclusions we see that the only day that appears once is Monday therefore this must be today as otherwise there would be more than one true statement.