What has the telephone done to us, or for us, in the hundred years of its existence? A few effects suggest themselves at once. It has saved lives by getting rapid word of illness, injury, or fire from remote places. By joining with the elevator to make possible the multi-story residence or office building, it has made possible -- for better or worse -- the modern city. By bringing about a great leap in the speed and space with which information moves from place to place, it has greatly accelerated the rate of scientific and technological changes and growth in industry. Beyond doubt it has seriously weakened if not killed the ancient art of letter writing. It has made living alone possible for person with normal social impulses by doing so it has played a role in one of the greatest social changes of this century, the breaking up of the multi-generational household. It has made the war chillingly more efficient than formerly. Perhaps though not provable (可证实), it has prevented wars that might have arisen out of international misunderstanding caused by written communication. Or perhaps--again not provable--by magnifying and extending irrational personal conflicts based on voice contact it impartially disseminates (传播) the useful knowledge of scientists and the nonsense of the ignorant, the affection of the affectionate and the malice (恶意) of the malicious. What is the main idea of this passage?