Some people give AI a very narrow definition as an applied sub-field of computer science. I prefer a definition that reflects the range of work reported at AI conferences, in AI journals, and the interests and activities of some of the leading practitioners, including founders of the subject. From this viewpoint AI is a very general investigation of the nature of intelligence and the principles and mechanisms required for understanding or replicating it. Like all scientific disciplines it has three main types of goals: theoretical, empirical, and practical.