IDEA, a term used both popularly and in philosophical ter minology with the general sense of "mental vision," (Gr. iS a connected with May, to see). To have no idea how a thing happened is to be without a mental picture of an occurrence. In this general sense it is synonymous with concept (q.v.) in its popular usage. In philosophy the term "idea" is common to all languages and periods, but there is scarcely any term which has been used with so many different shades of meaning. Plato used it in the sphere of metaphysics for the eternally existing reality, the archetype, of which the objects of sense are more or less im perfect copies. Chairs may be of different forms, sizes, colours and so forth, but "laid up in the mind of God" there is the one per manent idea or type, of which the many physical chairs are de rived with various degrees of imperfection. From this doctrine it follows that these ideas are the sole reality (see further IDEAL ISM) ; in opposition to it are the empirical thinkers of all time who find reality in particular physical objects (see HYLOZOISM, EMPIRICISM, etc.). For other meanings of the word see PSY CHOLOGY.