What does the brain do? The ancients thought it was a radiator, cooling the blood. Modern views see it as an activator, using inputs from the environment in combination with prior knowledge to generate behaviors (walking, talking, eating and drinking) and mental states (feelings, desires and beliefs). Recently the idea has emerged that the brain acts as a predictor, using inputs and stored knowledge to generate models of the world, and of the consequences of possible actions we and others might pursue. These models can predict what will happen in the next minute, hour or decade, and allow us to behave in the most adaptive way.