I am looking at you. How many things are necessary for this vision? First, the eyes. For if I am perfect in every other way, and yet have no eyes, I shall not be able to see you. Secondly, the real organ of vision. For the eyes are not the organs. They are but the instruments of vision, and behind them is the real organ, the nerve centre in the brain. If that centre be injured, a man may have the clearest pair of eyes, yet he will not be able to see anything. So, it is necessary that this centre, or the real organ, be there. Thus, with all our senses. The external ear is but the instrument for carrying the vibration of sound inward to the centre. Yet, that is not sufficient. Suppose in your library you are intently reading a book, and the clock strikes, yet you do not hear it. The sound is there, the pulsations in the air are there, the ear and the centre are also there, and these vibrations have been carried through the ear to the centre, and yet you do not hear it. What is wanting? The mind is not there. Thus we see that the third thing necessary is, that the mind must be there. First the external instruments, then the organ to which this external instrument will carry the sensation, and lastly the organ itself must be joined to the mind. When the mind is not joined to the organ, the organ and the ear may take the impression, and yet we shall not be conscious of it.