An Essay Concerning Human Understanding begins with a short epistle to the reader and a general introduction to the work as a whole. Following this introductory material, the Essay is divided into four parts, which are designated as books. Book I has to do with the subject of innate ideas. This topic was especially important for Locke since the belief in innate ideas was fairly common among the scholars of his day. The belief was as old as the dialogues of Plato, in which the doctrine of a world of ideas or universals had been expressed. Plato had taught that ideas are latent in the human mind and need only the stimulation of sense perception to bring them to the level of consciousness. Many of the philosophers of the so-called rationalistic school followed Plato in this respect.