David Hartley was an English philosopher and founder of the Associationist school of psychology.

Associationism is the idea that mental processes operate by the association of one mental state with its successor states. It holds that all mental processes are made up of discrete psychological elements and their combinations, which are believed to be made up of sensations or simple feelings. In philosophy, this idea is viewed as the outcome of empiricism and sensationism. The concept encompasses a psychological theory as well as comprehensive philosophical foundation and scientific methodology.