Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Key concept is undefined

The author tells you to exercise at certain intensity levels but nowhere in the book is "intensity level" quantitatively defined. Instead, he tells you that when you are winded enough you at the right intensity level.The book sets out guidelines for aerobic exercise using heart rate but heart rate does not equal intensity rate. You are supposed to figure "intensity rate" out for yourself by how distressed you get while exercising.Sort of like that famous US Supreme Court definition of obscenity: I know it when I see it.