This book defends the view that phenomenal states (qualitative or experiential states) are intrinsic states of the subject. Many current theories say that phenomenal states are relational states, especially of a functional or representational variety. The author argues that such theories cannot account for the important fact that phenomenal states can causally influence bodily movements. He captures this fact with a principle called the "activity constraint on experience", or the ACE: that if two experiences are distinct in their phenomenal character then they must be capable of causing distinct bodily movements. Relational theories entail that experiences can differ in their phenomenal character even though they cannot cause distinct bodily movements. We may therefore "play the ACE" against such theories and thereby reject them. No such theory can account for the ACE. A subjec&tgrave;s experience is therefore determined by the subject's intrinsic states.