Puzzle | Thief, Treasure and 2 Doors

Puzzle:

A thief has just found a pair of ancient treasure caves.

• One of the caves is filled with unbelievable treasure and the other has a fire breathing monster that will eat anyone
  who opens that cave.
• One cave has a black door decorated with diamonds and the other cave has a brown door decorated with sapphires.
• Each of the doors has an engraved description on top. The descriptions say:

  Black Door: Monster is here.

  Brown Door: Only One Door speaks the truth.

Which door should the thief open?

Solution:

Let us look at the description on the Brown door. It can be correct or wrong.

• Scenario 1: The description on the Brown door is true. Then the description on the Black door has to be true. That means that the inscription on the Black door is false and the cave with black door contains the treasure!
• Scenario 2: The description on the Brown door is false. Then either both the descriptions are false or both are true. Both cannot be true as that is impossible and not consistent. That means that both descriptions are false. Again, the treasure is in the Black door.