When Donald W. McConachie of Alexandria mailed his letter to me, he could not have known that yesterday's column would be about labor disputes. So the issues he raised in his letter were independently developed and articulated. He wrote:

"A recent news story said that the increase given to postal workers would raise their annual salary to $18,000. Someone at work mentioned that the garbage collectors in San Francisco were being paid $17,000 a year.

"I recall the discussions over the pay of the nurses at the Washington Hospital Center. They wanted $12,000 a year. I wonder what kind of priorities the people of this country place on different services they receive.

"People who need no special training, no special skills, are paid half-again as much as people to whom we entrust our lives. It doesn't make much sense to me.

"Nurses deal with life and death. They require extensive training and must pass exams and be licensed.

"We quibble about paying them $12,000 a year, but no one raises the slightest question about paying a postman $18,000 a year. Is it because we are afraid of the postal unions?

"Are these jobs really worth that kind of money? I don't believe they are. The electricians on the West Coast recently voted themselves a raise that will bring their wages to over $23 an hour. This amounts to over $47,800 a year. I wonder just what our priorities are."

When news stories refer to wages demanded or agreed to, it is not always clear whether the numbers mentioned refer to minimums, maximums, averages, or extrapolations. For examples, $23 an hour becomes $920 a week only if we are dealing with a 40-hour week, and $920 a week becomes $47,840 a year only if the worker is being paid for a 52-week year. But the fact is that some electricians have contracts that call for a 25-hour week, and heaven knows how many weeks of pay each contract guarantees per year.

Presumably there are seasons when electricians work 40 hours a week and collect time-and-a-half pay for the final 15 hours; possibly there are also seasons when there is only 25 hours of work about that. But I do know that wage comparisons among electricians, nurses, letter carriers and garbage collectors cannot always be made with precision.

I think it should also be noted that money was not an issue when nurses struck the Washington Hospital Center, nor in the settlement that followed. The nurses were given a 6 percent raise that guaranteed them "the highest nursing wages in the city" - with $13,384 the starting wage for a nurse with no experience and $18,917 the top wage for the most experienced head nurse in a unit. The nurses were disappointed that they couldn't win a union shop and other benefits, but the money package pleased them.

With that background, we can answer Donald's question: What are our priorities? Is a garbage collector entitled to more money than most nurses earn because his job is distasteful? Is a letter carrier entitled to more because his union has the strength to shut down the mails? Is an electrician entitled to $47,000 while a teacher gets $15,000? Should a pop singer be paying more income tax than the president of the United States earns in total salary?

Yes, in a free economy these times happen. Each person is entitled to sell his services to the highest bidder and to reject work that doesn't appeal to him. As wages rise, the work becomes more appealing. As your union gains strength, you can demand higher wages.

But if your union misjudges and demands more than the traffic will bear, employers will replace you with machines. In short, in our system, wages are set by the forces at work in a free marketplace.

As time passes, those forces wax and wane. Equilibriums are upset and new equilibriums take form. The locomotive engineer who was once among the best-paid of American workmen may now consider himself fortunate to be getting a regular paycheck of any kind as his railroad goes through bankruptcy proceedings. Today it is the airline pilot who makes more than a member of the Cabinet, but the day may come when he is replaced by a computer, and on the day the airlines will no longer fear a strike by pilots, they will welcome one.

The only constant in all this appears to be that athletes and entertainers will continue to prosper. No computer has as yet been taught to run back a punt or cause young women to swoon.