Like the painter Whistler, Prime Minister Margaret Thatcher works a lot with black and white. As Argentine generals and British miners can attest, she does not subscribe to the theory that right and wrong are too tangled to be sorted out. Lesser politicians are expected to speak peculiarly about, say, Soviet leaders. (Denis Healey, a former defense secretary, describing Mikhail Gorbachev's face: "Emotions flicker over a face of unusual sensitivity like summer breezes on a pond.") But Thatcher-watchers were puzzled by her statement during Gorbachev's December visit: "I like Mr. Gorbachev. We can do business with him."

She is cross about television coverage that took those words out of the context of words about the chasm between East and West. But she has been in politics long enough to know the risks of such excerptable statements. Besides, the question is: What business does she think Gorbachev is in?

Her answer, given in an interview at No. 10 Downing Street, is interesting: Gorbachev, like any Soviet leader, is in the business of making statecraft conform to a "science" that is in shambles.

She recalls a Chernenko speech "all about aren't they lucky: They are the only country that has a scientific form of government." Faced with the system's failure to supply material needs, "They say it's because people are not working it right. . . . And now if you look at Mr. Gorbachev's acceptance speech, you will find almost the pure Andropov-Chernenko doctrine in it: Our system is not producing the results, we must be much more disciplined, we must have some more initiative, and there must be no deviation."

Soviet leaders feel, she says, they are custodians of an ideology that they are certain is correct, so the system must work. But it does not. The explanation, they feel, must be that people are not working or behaving properly.

If her analysis is correct, Soviet "reform" means not abandonment of ideology but rededication to it, and ruthless removal of human failure that prevents the system's success. Thus it is interesting that in statements made shortly before and after assuming power Gorbachev has promised "resolute measures" to purge "moral degenerates" and other "alien phenomena" from Soviet society. There will be "consolidation of labor" and a fight against "any manifestation of showiness and idle talk, swagger and irresponsibility." Presumably, while he was making these statements, his pond-like face flickered sensitively.

Thatcher recalls being in Yugoslavia and having housewives tell her they were short of detergent. "I said good heavens. . . . What are you doing bothering yourselves about that? Tell your grocers to go out and buy some. It's your grocers' job." They said: "We don't make any." Thatcher said: "There's plenty overseas." They wondered what they could use for foreign exchange.

That should have taught Yugoslav housewives not to go to Thatcher seeking sympathy about the detergent shortage. She stamps her foot, figuratively speaking, at the folly of command economies. She believes the contrast with the bounties produced by market economies is so instructive that the West should work to get Soviet leaders into "Bloomingdale's or Saks Fifth Avenue" or, she says patriotically, "Marks & Spencer."

This idea is a version of something to which the West, and not least the Reagan administration, is addicted: didactic diplomacy. The plan is to tame the Soviet elite by teaching it a thing or two. For example, we negotiate in the Helsinki proc "rules" of civilized behavior.

Didactic diplomacy -- reform of Soviet behavior through remedial education -- is one of many reform plans the West has conceived. Another was bribery, sometimes called d,etente: We would make getting along with us profitable. Another plan is diplomacy as psychotherapy, with two approaches.

One is the "threat-perception theory." According to it, the spring driving Soviet aggressiveness is insecurity, so U.S. policy should be to prove that we mean no "harm" (President Reagan's word). Another is the "frustration-aggression theory." According to it, if the Soviet Union is not frustrated (in its quest for "respect," or arms "parity," or whatever), it will not vent frustrations in aggression.

Thatcher's idea of a diplomacy that is at once prudent and didactic is: Keep your powder dry and keep exposing the Soviet elite to the larger, more successful world. Unfortunately, that elite is comfortably supplied with material goods and would lose its caim to privilege were it to put aside the "science" of which it is custodian. The elite will not commit a grand act of self-liquidating altruism, de-mystifying and de-legitimizing themselves merely to improve the material life of the masses.

Furthermore, given what that science says about the inevitable death struggle with capitalism, the conclusion dictated by Thatcher's analysis is: Keep your powder dry, and have lots of it.