Many of us have a vague feeling that things are moving faster. Doctors and executives of ten complain that they can't keep up with the latest developments in their fields. Hardly a meeting or conference takes place today without some talk about 'the challenge of change'. Among many there is an uneasy mood—a suspicion that change is out of control. Not everyone, however, shares this anxiety. Millions sleepwalk their way through their lives as if nothing had changed since the 1930's, and as if nothing ever will. Living in what is certainly one of the most exciting periods in human history, they attempt to withdraw from it, to block it out, as if it were possible to make it go away by ignoring it. One sees them everywhere. Old people, resigned to live out their years, attempting to avoid, at any cost, the intrusion(扰乱，侵犯) of the new. People of thirty-five and forty five, nervous about student riots, sex, L.S.D, or miniskirts, feverishly attempting to persuade themselves that, after ail, youth was always rebellious, and that what is happening to day is no different from the past. Even among the young we find an incomprehension of change: students so ignorant of the past that they see nothing unusual about the present. The disturbing fact is that the vast majority of people find the idea of change so threatening that they attempt to deny its existence. Even many people who understand intellectually that change is accelerating, do not take this critical social fact into account in planning their own personal lives. What is the main idea of the passage?