Association of ideas, or mental association, is a process by which representations arise in consciousness, and also for a principle put forward by an important historical school of thinkers to account generally for the succession of mental phenomena. It is used mostly in the history of philosophy and of psychology. One idea was thought to follow another in consciousness if it were associated by some principle. The three commonly asserted principles of association were similarity, contiguity, and contrast, numerous others had been added by the nineteenth century. By the end of the nineteenth century physiological psychology was so altering the approach to this subject that much of the older associationist theory was rejected.