Human nature being what it is, a cardinal rule of life ought to be: Anything that sounds too good to be true, usually is.

That point should be kept firmly in mind in assessing a purported pledge this week from the Kremlin.In winding up a visit to Russia this week, West German Chancellor Helmut Kohl announced that the Kremlin has promised to release all political prisoners by the end of the year and support a human rights conference in Moscow.

If Soviet leader Mikhail Gorbachev follows through on this promise, it could constitute one of the most dramatic signs yet that he's serious about breaking with Russia's long history of human rights abuses.

But at this point a healthy dose of skepticism is still in order.

Keep in mind that when U.S. Secretary of State George Shultz visited Russia several months ago, the Soviets promised to release 48 political prisoners. But only 11 of those 48 were actually released.

Keep in mind that the Kremlin has not confirmed Kohl's report and that there are major disagreements about the number of political prisoners in Russia. According to Kohl, the Soviets are talking about releasing all persons the West considers political prisoners. But the Soviets say there are only a dozen such prisoners. The U.S., however, lists 234 of them, while Soviet dissidents say the true number is nearer 400.

Keep in mind, too, that even after the last political prisoner has been freed, Russia's hands will still be far from clean. The Soviets regularly harass dissidents, keep Jews and others from emigrating, and throw people into insane asylums just for opposing the Kremlin.

In fairness, Gorbachev deserves some credit for having released some 140 political prisoners since he took over six years ago. But the fact remains that Russia is still the world's leading violator of human rights and often seems far more interested in making promises just for their propaganda value than in making fundamental reforms.