In practicing concentration of voluntary attention, it should be remembered that concentrating consists not only of _focusing_ the attention upon a given object or subject, but also of the _shutting out_ of impressions from other objects or subjects. Some authorities advise that the student endeavor to listen to one voice among many, or one instrument among the many of a band or orchestra. Others advise the practice of concentrating on the reading of a book in a room filled by persons engaged in conversation, and similar exercises. Whatever aids in _narrowing the circle_ of attention at a given moment tends to develop the power of voluntary attention. The study of mathematics and logic is also held to be an excellent practice in concentration of voluntary attention, inasmuch as these studies require close concentration and attention. Attention is also developed by any study or practice which demands _analysis_ of a whole into its parts, and then the _synthesis_ or building up of a whole from its scattered parts. Each of the senses should play a part in the exercises, and in addition to this the mind should be trained to concentrate upon some one idea held within itself--some mental image or abstract idea existing independently of any object of immediate sense report. CHAPTER VI. Perception. It is a common mistake that we _perceive_ everything that is reported to the mind by the senses. As a matter of fact we perceive but a very small portion of the reports of the senses. There are thousands of sights reported by our eyes, sounds reported by our ears, smells reported by our nostrils, and contacts reported by our nerves of touch, every day of our lives, but which are not _perceived_ or _observed_ by the mind. We perceive and observe only when the attention, reflex or voluntary, is directed to the report of the senses, and when the mind interprets the report. While perception depends upon the reports of the senses for its raw material, it depends entirely upon the application of the mind for its complete manifestation.