The apperceptive mass was part of theory of psychic mechanics proposed by Johann Friedrich Herbart (1776-1841) to account for the workings of the mind. Herbart believed that the basic units of the mind are ideas. He agreed with the empiricists that they come from experience. He departed from their view concerning what happens once ideas enter the mind. Herbart claimed that at that point ideas have a life all of their own, and thereafter are never destroyed or completely forgotten. Moreover, ideas strive for conscious expression through a process he referred to as psychic mechanics. At any given moment, similar ideas attract and form an apperceptive mass in consciousness. Ideas incompatible with the mass are repelled by it (repression). The repressed ideas are relegated to the unconscious mind where they remain, either until a more compatible apperceptive mass emerges, or until enough there are enough similar repressed ideas to form their own apperceptive mass. Herbart used the term limen to refer to the border between the unconscious and the conscious mind. Aiming to do for the mind what Isaac Newton (1642-1727) had done for the physical world, he tried to express mathematically the relationships among the apperceptive mass, the limen, and the conflict of ideas.