"The American psychologist William James discussed consciousness at some length. In his monumental work The Principles of Psychology... he described five properties of what he called "thought." Every thought, he wrote, tends to be part of personal consciousness. Thought is always changing, is sensibly continuous, and appears to deal with objects independent of itself. In addition, thought focuses on some objects to the exclusion of others. In other words, it involves attention. Of attention he wrote, in an oft-quoted passage: "Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several possible objects or trains of thought. ...It implies withdrawal from some things in order to deal effectively with others."