Finally, the fourth proposal was that there may also be linguistic universals underlying all human languages which could be exploited to make the problem of translation more straightforward. Weaver argued for this position with what is one of the best-known metaphors in the literature of machine translation: "Think, by analogy, of individuals living in a series of tall closed towers, all erected over a common foundation. When they try to communicate with one another, they shout back and forth, each from his own closed tower. It is difficult to make the sound penetrate even the nearest towers, and communication proceeds very poorly indeed. But, when an individual goes down his tower, he finds himself in a great open basement, common to all the towers. Here he establishes easy and useful communication with the persons who have also descended from their towers".