FIVE YEARS AGO, THE PATIENT WAS IN A CAR CRASH. The accident fractured his skull and damaged his brain. Since then, his mind has seemed devoid of thought. His eyes are open. But if you talk to him, he doesn’t answer, and if you try to get his attention, he seems unaware of your presence. Except for simple reflexes such as opening his eyes and breathing, he doesn’t even move. His diagnosis? “Persistent vegetative state.” A team of medical specialists evaluated him for a month and could find no sign of human thought.

Then some other doctors had a different idea. They knew that, from the outside, his mind seemed blank. But how did things look “from the inside”? To find out, they put him in a brain imaging device. While taking pictures of his brain, they asked him to imagine two everyday activities: hitting some tennis balls and walking from room to room in his house. Next, they asked him simple yes/no questions—for example, “Do you have any brothers?”—and told him to think about tennis playing if the answer was “no” and to think about walking through his house if the answer was “yes.”

If you had observed his doctors, their behavior would have looked futile. Why ask questions of a man who has seemed completely without human thought for five years? You would expect the brain images to be blank. But they weren’t! When the man was asked to imagine playing tennis and walking through his house, his brain activity resembled that of perfectly normal, healthy people who had been asked to think about those same activities (Monti et al., 2010). When the doctors checked his answers to the yes/no questions, they found that he got them right. He was no vegetable. He could hear the doctors’ voices, understand their questions, and answer them correctly.

His injuries had robbed this man of many abilities: walking and talking, smiling and frowning, even nodding or shaking his head. Yet there is something he still had: consciousness.

WHAT ARE YOU experiencing right now? You’re thinking about the sentences on this page. You might be feeling the temperature of the room you’re in (especially if it’s too warm or too cold), the pressure of a chair against your skin (especially if it’s uncomfortable), or the sound of some nearby music (especially if it’s loud). You might be experiencing hunger, fatigue, sleepiness, or maybe a feeling of interest as you start a new chapter of this book.

These external stimuli and internal feelings—temperature, sound, fatigue, and so forth—are stimuli of which you are conscious. Consciousness is your awareness and personal experience of your surroundings and yourself.