In his Observations on Man, his Frame, his Duty, and his Expectations (1749), physician David Hartley expounded a physiological theory of "vibrations" to explain his conviction that the moral sense was not inborn, but rather a consequence of the association of ideas. Particularly after his work was popularized by Joseph Priestley in his abridgment Hartley's Theory of the Human Mind on the Principle of the Association of Ideas (1775), Hartley's ideas exerted broad influence on literature, philosophy, medicine, psychology, and issues such as education and reform.