What carries messages around the body? What is the physical mechanism of memory? Today, we unthinkingly assign such tasks to the electrical nature of the nervous system. But erstwhile common sense was equally sure that ‘animal spirits’ coursed through our bodies and brains, transmitting information. For many centuries, most of the Western World believed that animal spirits governed not only sensation, but also memory, imagination, belief, passion and health. When we are dispirited, in good spirits, or giving a spirited reply, we are using the metaphors of a theory that held sway from Ancient Greek times until 200 years ago 1 – a theory now forgotten.