All we really know is that men, unlike animals, somehow invented certain sounds to express thought and feelings, actions and things, so that they could communicate (30)____ each other; and th

All we really know is that men, unlike animals, somehow invented certain sounds to express thought and feelings, actions and things, so that they could communicate (30)____ each other; and that later they agreed upon certain signs, called letters, which could be combined to present those sounds, and which could be written down.