WASHINGTON — It was the best of times, it was the worst of times. That judgment of Charles Dickens on the per iod of the French Revolution in his great novel, ‘“‘A Tale of Two Cities,’’ says something about our revolutionary time. Man has the knowl edge, the skills, the technology to trans form the world. The vista of a new life has a radiance that glistens just beyond our grasp. And at the same time we know that with each passing month the gap between the po tential of the best of times and the reality of a time of decay and decline grows wider. The old prescriptions fail, the old tags of ‘‘liberal’’ and ‘‘conservative’’ seem as irrelevant as the incantations of a Bantu medicine man. This is what a dis tinguished thinker on urban problems is saying in a forthcoming book with a high ly explosive charge. In a recent broad cast Eric Sevareid said that advance copies already circulating of Edward C. Banfield’s ‘‘The Unheavenly City’’ were already making sizzling sounds as the fat hits the fire. Banfield, a professor of urban gov ernment at Harvard, sets out to punc ture the comfortable assumptions that many politicans and commentators live by. He is scornful of what the con temptuous call the knee-jerk liberal re action the compulsion to do good, an in tegral part of the puritan inheritance, is, he suggests, more likely to be harmful rather than helpful. His book bristles with brilliant paradox, ‘Training programs, ‘ he writes, ‘‘do not as a rule offer any solution to the problem of hard-core unemployment be cause the same qualities that make a worker hard-core, also make him un able or unwilling to accept training. More generally, giving lower class persons ‘really good’ jobs is not a feasible way of inducing them to change their style of life, because that very style of life makes it impossible to give them ‘really good’ jobs.’’ Banfield has his own definition of pow er and lower class in our supposedly classless society. He says that a per son who is poor, unschooled and of low status may be upper class if he is psy chologically capable of providing for a distant future. Correspondingly one who is rich and member of the ‘‘400'’ may be lower class if he cannot control his im pulses and must, therefore, live from moment to moment. A general welfare program, Banfield holds, would put an immediate end to poverty. But, by causing the breakup of a great many families, by reducing motiva tion to work and to save in general to take some account of the future and by discouraging migration from areas of poor opportunity to areas of good op portunity, it would in the long run do more harm than good. Making heroin and other addictive drugs readily available on prescription would reduce violent crime in New York and some other large cities by reliev ing the addict of the necessity of steal ing in order to support his habit. But the effect would also be to increase the number of addicts and the resulting harm would likely be greater than the benefit from the reduction in violent crime. Banfield has been a White House con sultant. He was chairman last year of the task force on model cities. This ap pointment is said to have resulted from his close relationship with Arthur Burns, who recently left the White House to be chairman of the Federal Reserve Board. At Harvard Banfield was a friend and colleague of Daniel Patrick Moynihan, counselor for establishing a world’s rec ord as a memoranda writer. Of Ban field Moynihan says: You may disagree with him but you had better be prepared to say why you disagree.’’ Echoes of Moynihan’s memorandum on Negro progress and the need for a period of ‘‘benign neglect’’ to allow tensions to subside can be heard in the Banfield book. The suspicion will not doubt that, with the problems of the cities seeming so insoluble, there is a need to rationalize why they do not yield to the long-term ef forts lumped under the familar rhe toric of housing, education, job training, etc, etc. But, for all that, it is stimulating to have a recognized authority say that the tired old emperor to whom we have so long given allegiance has no clothes. (Copyright, 1979, by United Feature Syndicate, Inc.)