But how can we study something as complex and mysterious as the mind? Even if we were to split open the skull of a willing volunteer and have a look inside, we would only see the gloopy grey matter of the brain. We cannot see someone thinking. Nor can we observe their emotions, or memories, or perceptions and dreams. So how do psychologists go about studying the mind?

In fact, psychologists adopt a similar approach to scientists in other fields. Nuclear physicists interested in the structure of atoms cannot observe protons, electrons and neutrons directly. Instead, they predict how these elements should behave and devise experiments to confirm or refute their expectations.