Appearing on "Meet The Press" recently, Richard Darman of Treasury (sounds like an old radio serial) was asked why President Reagan had not used tax simplification as an opportunity to do other things as well -- such as reduce the deficit. Darman, a deputy secretary and a former White House aide like his boss, James Baker, responded by enunciating the Doctrine of Separability. It's a fancy way of saying the president wants to have things his way.

Officially, separability is supposed to mean nothing of the sort. It is meant to endow the tax plan with a sort of retroactive virginity, make it seem so pure and so above political considerations that it would be wrong, maybe even a sin, to suggest that the tax code be reformed for a purpose -- such as making the poor a little less so. Perish the thought, say Darman and others. The Doctrine of Separability is inviolate.

But Ronald Reagan has not written a tax plan that is "separable" from his own political program. Instead, it advances it. It's true Reagan has had to swallow his longtime opposition to corporate income taxes, and it's true also that he has taken away the toys of the rich -- most tax shelters and $100 expense account lunches. But these concessions to political reality are mere asterisks to what all along has been his larger political purpose -- reducing the size of government by starving it for funds. The Reagan tax plan is no departure from that goal. Indeed, aspects of it extend the president's reach. Some state and local governments will probably now shrink as well.

To this, Democrats and Republicans alike seem to be saying of the president, "He's entitled." Most Republicans share the president's political values. But what about Democrats -- especially those who are supposed to differ with the president on the obligation of government to the poor? Aren't they also entitled to have a tax plan express their political values? Where is it written that only the president gets to play politics with the tax code?

It must be written somewhere, because you cannot find five Democrats who will stand up and say they will not crucify Democratic political values on the cross of tax simplification. They all seem to think that simplifying the tax code is so important that it takes precedence over anything else -- the budget deficit, the rape of the poor, the plight of urban blacks.

In fact, when Patrick Buchanan called New York Gov. Mario Cuomo a no-account redistributor of income, no one in the whole Democratic saloon had the nerve to remind Buchanan that that's one of the things the progressive income tax is supposed to do. Not surprising. Almost no one has asked why the tax break that is now going to make the rich richer cannot be used to help those of the poor who have become poorer under President Reagan.

Less than one week after the president crooned his tax plan to the nation, a response came in the mail from the Children's Defense Fund. It made awful reading. To be a black child is to be consigned to a special kind of American hell. It means being twice as likely as a white child to die before the age of one, to live in substandard housing, to see a parent die, to have unemployed parents, to live in an institution, to be born to a teen-ager and to exist on welfare programs ravaged by inflation and inattention.

Of course, money alone is not the solution to this festering concoction of social pathologies. But without money -- without federal programs that cost money -- hitting "the number" is about the only hope some poor people have. And yet there are precious few in either political party who point toward these black children and then to the tax bill and ask why people making $200,000 a year should get a $5,000 tax reduction and the kids get nothing.

It's probably true that the more you monkey with the tax bill the less chance it has for passage. That's a shame, because there are elements in it that are worthwhile. But there is no separating it from either the fiscal problems of the nation or the plight of the poor. Separability is a wonderful doctrine if you're going to get something out of the president's tax program. Otherwise, it's just another word for cruel indifference.