The problem of the Stone Age: how to get food and females?
The problem of the ancient world: how to stop the barbarians?
The problem of the Middle Ages: how to punish pagans and do nothing?
The problem of the Renaissance: how to move to art and forget about the Middle Ages?
The problem of the 20th century: how to stop the conflict between the countries of the first and second world?
The problem of modernity: what would look like under the canvas?