HOW DO YOU LIBEL Philadelphia? No, really. The vexing question must now be answered since, to the astonishement of all, the city parents of Philadelphia have decided to forge ahead with their lawsuit against The Washington Post and are threatening at least one syndicated columnist Carl Rowan -- for libeling the city. The issue in our case is a story that appeared in conjunction with the government's suit against the Philadelphia police department for alleged systematic policy brutality. We trust you will forgive us. however, if, as is only becoming in the circumstances, we avoid taking a stand on the actual merits of the case (except to note that we are right and they are wrong), and go on to the larger, philosophical, if-a-tree-falls-in-the-forest issue that have been raised by this enterprising action.

Libel? Philadelphia? How and where do you begin? Do you attach the estate of the late W.C. Fields until such time as his legatees have compensated the city for damages flowing from his famous gravestone legend ("On the whole, I'd rather be in Philadelphia")? What about the ancient gag that has first prize in the contest being a week in Philadelphia, and second prize being two weeks there? Mind you, we have nothing against Philadelphia. Some of our best friends are Philadelphians. We even have a certain sympathy, since we are not ourselves unfamiliar with the sorrows of being an oppressed metropolitan area. When was the last time you heard the work "Washington" used in a non-libelous way by people who don't live here? What's that you say -- never? Would you mind deposing to the effect?

But there we go talking pre-trial testimony again, when all we had in mind to do was explore the truly gripping metaphysics of the thing. For example, the Philadelphia city solicitor has told the U.S. District Court that $20 million will pretty much make up for the damage the city believes has been done it. Does that figure seem high? Low? About right? Can any amount of money this side of Persian Gulf emirates make up for the things that people have been saying, profligately and interminably, about Philadelphia all these years? Do you realize that the defamation has gone so far that even to refer to this city solicitor as a "Philadelphia lawyer," which he surely is, would be to risk adding to the gravity of the offense? Our own sense of it is that money is useless coin in so dire a situation and that libel suits are beside the point: the only way the rest of us can make it up to Philadelphians is to say something good about their city -- just as President Carter, wherever he may be, once suggested we do about the country as a whole.

So here goes. One of the good things about Philadelphia is that its Mayor, Frank Rizzo, is soon to be a former mayor. The Philadelphia electorate refused to renew his leas last spring. This is not just good, it is great. Before going back to the arduous task of defending the poor, miserable slandered city of Washington from its detractors high and low, we would like to congratulate the people of Philadelphia for this example of eminent good sense.