PERHAPS President Reagan's lifting of the grain embargo is an aberration, a one-shot exception to a general policy of considering matters relating to the Soviet Union as parts of a strategic whole. This makes it quaint, even a bit touching, that he should honor a campaign promise by taking a step that cuts so embarrassingly across the main thrust of his approach to Soviet power.

The scale of the administration's embarrassment remains immense: President Reagan is helping Moscow out of a grain pinch, breaking faith with the Afghans and the Poles, setting a nothing-for-something precedent in diplomacy, announcing that he caves to domestic pressure groups, and licensing all manner of other would-be exporters, American and foreign, to try to sell to Moscow what they will. Still, if the lifting of the embargo is the exception that proves the rule of American strategic determiniation, all is not lost.

It occurs to us, however, that there is another possible explanation for Mr. Reagan's decision. Perhaps he does not regard his anti-embargo assurances to the farmers so much as a "campaign promise" as an expression of a deeply felt free-market philosophy that disposes him to resist controlling normal civilian commerce, however that might be defined. This would lead not to a transient or accidental contradiction but a permanent one between his economic policy and the demands of a prudent conventional foreign policy designed to contain Soviet expansion. It would be, in our view, a politically costly and strategically distracting contradiction, the more so for being witting and continuous. Can it be that this is what Mr. Reagan has in mind?

If he is at all inclined in that direction, there is yet another factor that must be worked into the equation. Jimmy Carter imposed the partial grain embargo and took related steps at the time of the Soviet invasion of Afghanistan in order to bring to bear on the Soviet Union a range of sanctions at the peaceful end of the spectrum. The idea was that, in this instance, as serious as it was, peaceful measures were to be preferred over others more toward the military end of the spectrum. The idea behind that was essentially gradualism: responding to reprehensible Soviet behavior by starting small, taking one step at a time, adding pressures as necessary and feasible, making Soviet aggression costly, bringing the allies along, giving diplomacy time to work, playing by the rules. This concept has been applied by successive American administrations in all situations where a recourse to force has not been thought necessary, and even in some situations where it has.

Now comes Ronald Reagan, who is taking out of his own hands -- conceivably, not just in this incident -- the principal lever, trade, available for peaceful and gradual response to Soviet actions of which the United States disapproves. By doing this he is pointing himself toward, and to a degree committing himself to, a whole other manner of response, one in which he would conceivably reply to the Soviet Union more abruptly, more forcefully, more "effectively" and in a more unpredictable and unorthodox way.

The lifting of the embargo could be the opening signal of a startling and radical new approach to Soviet power in which the perceptions and risks on both sides would be quite different from what they have been until now. There have been hints of this between some of Mr. Reagan's lines but nothing of real substance. It will be interesting, not to say surpassingly important, to see if this is what the president really has in mind -- changing the rules of the game -- so that others can fairly discuss and judge it.