Once there were three great Logic Masters, and one day they had a disagreement about which of them was the wisest. So they went to the Grand Master of Logic and asked him to resolve their argument.

“OK,” said the Grand Master. “I will blindfold you and paint a red or a blue dot on each of your foreheads. when I take the blindfolds off, raise your hand if you can see at least one red dot. The person who guesses the colour of the dot on their forehead correctly is the wisest of all of you.”

He painted a red dot on each of their foreheads. Then he took the blindfold off and all three Logic Masters raised their hand. They sat in silence for a while, thinking. At last, one man said, “I have a red dot on my forehead.”

How did he know?