Two conditions must be fulfilled before external vibrations can become audible. You hear me speak because there is no wall between you and myself. A wall prevents communication. Then when a person is speaking out in the open with the same pitch of voice as I use at this moment, you cannot hear his voice as well as mine, for the house we are in gives the sound a place to echo in and become clearly audible. Thus these are the conditions: first a current must be established, a channel or opening through which the sound or the words can reach another person; and secondly the sound must not be able to scatter in all directions, but it must be directed and concentrated towards the other, so that it can reach the inner or mental process which we call thought.