The thing about Ronald Reagan that most troubles foreigners -- and one that troubles a lot of Americans, too -- is his China syndrome. I refer not to a meltdown of a nuclear reactor, but to the Republican presidential candidate's tendency to say, as he has said notably about China, what is no less earnest than it is foolish, not thought through. This is something quite apart from the general slant of his views.

Of China, Reagan has said that he would restore official ties with the Republic of China on Taiwan, with which the United States ended diplomatic ties as the main condition of normalizing relations with the People's Republic of China. Evidently Reagan has a great admiration for the staunchly anti-communist government on Taiwan. He feels that the Carter administration wrongfully abandoned Taiwan, and broadcast signals of inconstancy worldwide, when it completed President Nixon's opening to Peking last year.

All this is true, in a sense. But only in a sense. It does not seem to bother Ragan that to restore official ties with Taipei would, among a range of effects, undermine the relationship the United States has built up with Peking -- or so Peking has clearly warned. The Washington-Peking connection is central to the preferred Reagan strategy of containing Soviet power. But he would multiply the difficulty of pursuing that strategy if he were to turn back the clock to recognition of Taiwan.

Nor does it seems to bother Reagan that his proposal, which calls for adding recognization of Taipei to the existing recognition of Peking, is emphatically opposed in both places. Neither likes the support Reagan gives to the one idea that Chinese of all persuasions find noxious, the idea of two Chinas -- a concept, remember, that the United States explicitly rejected in Nixon's Shanghai communique.

In the event, the damage may be limited. Reagan's remarks on Taiwan seem to have invigorated some of the GOP's old-time partisans of Chiang Kai-shek, and they may yet be heard from at the Republican convention. But Reagan's lieutenants can count on the backing of both the strategic-minded and the trade-minded among his constituents, and they are trying discreetly to deflate the issue. They murmur that restoration of ties with Taiwan is not the candidate's real position. The impression is conveyed that no responsible administration would want to go back and reopen the questions already decided with Peking.

Perhaps so. But on China policy, Reagan is under a double shadow. Though a Republican administration made the opening to Peking, Reagan's advisers are drawn mostly from the wing of the party known for its reflexive anti-communism and specifically for its pro-Taipei bearings. One adviser, William Van Cleave, in a 1968 article entitled "Assertive Disarmament," recommended a strike to take out China's nuclear facilities. At the same time, the ambivalence to which the Reagan camp has retreated under pressure from Peking and from GOP moderates has left an uncertainty about just what his policy would be.

Nor is this the end of the matter. Though Reagan's admirers solicit respect for their man's readiness to give simple answers to questions that sober other people, what is more evident to me is his disposition to believe that the world is what he says it is, a relatively uncomplicated place open to being made over at his call. Reagan seems to operate on a level of instinct or fundamental values and, casually and genially, to push past facts and contradictions that, rightly, slow others down. You culd say that the discipline of responsibility, plus long naps, will ease that problem, but such factors tend to put power in the hands of advisers -- and in Reagan's case that is not an entirely comforting prospect.

Meanwhile, Reagan neglects what might safely and wisely be done to show real extra benefit, not just a semantical boost, to Taiwan: to encourage further progress within the unique legislative framework that was set up to replace the diplomatic framework that had previously guided U.S.-Taiwan relations. Sen. John Glenn, reviewing the record, has just reported that the first year went "remarkably well" in ensuring the prosperity and security of Taiwan. No doubt it could work even better. Some experts, noting Peking's new receptivity to Taiwan-type modernization and Taiwan's hints of interest in helping Peking's economic "Taiwanization," would like to see Washington push the pace.

But this is precisely the type of common-sense diplomacy that is precluded when Reagan's China syndrome comes to the fore.