THE BROAD DECLINE in the producers price index is another welcome sign that the inflation is slackening. But there's no reason to think, unfortunately, that the process isn't reversible. This decrease in the inflation rate doesn't seem to be owed to any great change in the structure of the economy, or public attitudes, or the wisdom of public policy. It is mainly the result of an onerous recession, which in turn is the result of high interest rates.

Suppose that the government were to find some magic spell to reduce interest rates without changing anything else in the economy, and a strong recovery got under way. What would happen to inflation rates? The evidence indicates that the past months' painful progress would soon be lost, and prices would begin to accelerate again.

The administration applauds these latest figures, as well it might. But they are not a signal that the Reagan strategy is working. The Reagan strategy was an attempt to do the unprecedented--to make inflation slow down while economic growth speeded up. The deflationary half of the president's program was at war with the expansionary half, and deflation won. The economy is now going through the familiar wringer.

Prices have ceased their rapid rise, and many prices--farm prices are a conspicuous example--are falling sharply. But there was never any doubt that a sufficiently severe recession could achieve that result. The greater challenge, now as a year ago, and 10 years ago, is to find ways to hold inflation down effectively without high unemployment and waves of bankruptcies. Or, to put it the other way around, is there a way to manage a recovery without setting off a new round of inflation, like the one following the 1975 recession? The administration still hasn't got an answer to that one.