Pleasure describes the broad class of mental states that humans and other animals experience as positive, enjoyable, or worth seeking. It includes more specific mental states such as happiness, entertainment, enjoyment, ecstasy, and euphoria. The early psychological account of pleasure, the pleasure principle, describes it as a positive feedback mechanism, motivating the organism to recreate in the future the situation which it has just found pleasurable and to avoid situations that have caused pain in the past. The experience of pleasure is subjective and different individuals will experience different kinds and amounts of pleasure in the same situation. Many pleasurable experiences are associated with satisfying basic biological drives, such as eating, exercise, hygiene, sex or defecation. Other pleasurable experiences are associated with social experiences and social drives, such as the experiences of accomplishment, recognition, and service. The appreciation of cultural artifacts and activities such as art, music, dancing, and literature is often pleasurable. In recent years, significant progress has been made in understanding the brain mechanisms underlying pleasure. One of the key discoveries was made by Kent C. Berridge who has shown that pleasure is not a unitary experience. Rather, pleasure consists of multiple brain processes including liking, wanting and learning subserved by distinct yet partially overlapping brain networks. In particular, this research has been helped by the use of objective pleasure-elicited reactions in humans and other animals such as the behavioral ‘liking’/‘disliking’ facial expressions to tastes that are homologous between humans and many other mammals. Recreational drug use can be pleasurable: some drugs, illicit and otherwise, directly create euphoria in the human brain when ingested. The mind's natural tendency to seek out more of this feeling (as described by the pleasure principle) can lead to dependence and addiction. Berridge and Robinson have proposed that addiction results from drugs hijacking the ‘wanting’ system through a sensitization of the mesolimbic dopamine system.