We think all the time. A life devoid of thought would not be recognizably human. It is one thing to think and another to understand thinking. ‘What is thought?’ considers this question by looking at the study of thought in philosophy, psychology, neuroscience, and linguistics. The faculty of thought raises a number of questions. What does the possession of such a faculty require? Which creatures use this faculty? What is the relationship between the faculty of thought and other mental faculties? Thought can refer to a certain type of mental state or event. It is also an activity. What are the differences between one type of thought and another?