MUCH OF WHAT we do goes on outside the pale of consciousness: whether we adjust our body posture or decide to marry someone, we often have no idea why or how we do the things we do. The Freudian notion that most of our mental life is unconscious is difficult to establish rigorously. Although it seems easy to answer the question “Did you (consciously) see the light turn on?” more than 100 years of research have shown otherwise. The key problem is defining consciousness such that one can measure it independently of the internal state of an individual’s brain while still capturing its subjective character.