Here's a thought experiment: I have invented a machine I call the Expero. You strap yourself inside, like an astronaut into a rocket, except the Expero takes you to someone's inner space, not outer space. You get to choose a person, called the Objective – maybe a friend, or a lover, or a celebrity, or a perfect stranger. The Expero personnel painlessly, safely, and surreptitiously slip tiny electrodes into the Objective's brain and heart while the Objective continues going about her everyday business as if nothing had happened. When you, safely in the Expero, push a button, the electrodes transmit to you in high fidelity a moment of the Objective's thoughts, feelings, and sensations – you get to think, feel, and sense exactly what she happened to be experiencing at the instant you pushed the button. Push the button again and you get another helping of her inner experience. The thought experiment is: Would you queue up for a chance to ride the Expero?

This book is for those who answer yes. The Expero doesn't exist, sorry to say, but the idea that the Expero would lead to fascinating discoveries exists widely. People have been fascinated by inner experience ever since there were people: The earliest known writings (Gilgamesh, the Iliad) described the thoughts and feelings of the characters. Psychology was founded on the attempt to “introspect” the contents of consciousness, an attempt so problematic that the mere mention of “introspection” or “consciousness” became psychological heresy for most of the twentieth century.