As a Noun: 1. Activity in the mind. English = thinking. 2. The study or creation of theories about basic things such as the nature of existence or how people should live; or a particular set of such ideas. English = philosophy. As a Verb or Adjective: 1. To think about something for a long time, often before deciding what to do. English = mull over. 2. To have ideas in the mind. English = think. 3. To study or create theories about basic things such as the nature of existence or how people should live. English = do philosophy, study philosophy.