The Mexicans are good people and good neighbors, and their special heritage and situation must be taken account of in American policy-making -- and all that. But the Mexicans can be very difficult in some matters affecting American interests and in none more so than El Salvador and the whole question of revolution in Central America. The case in point is the foreign minister's reaction to the resumption of American military aid to El Salvador. The Salvadorans, he said, must "solve their own problems." Otherwise, the bloodshed will increase and "the conflict will inevitably become an international one."

To the left, it is an old story that help arriving with, in this instance, a Nicaraguan-Cuban-Soviet pedigree is legitimate and natural, while help arriving from the United States is "international" and interventionist. Ordinarily, perhaps, one might accept this double standard as irritating but relatively harmless. The Mexicans enjoy thinking of their society as the product of just such an ongoing revolutionary process as is said to be sweeping Central America. Many Mexicans tend to pose the acceptance of the legitimacy of Mexico's ideologilcal anti-Americanism as a test of American political maturity. Some go on to contend that they know better than Americans how to live with revolution -- by ingratiating, enveloping and ultimately smothering it.

But something is missing from the common Mexican prescription. It is bound to become more noticeable as the crisis in Central America, and Mexico's oil-fed appetite for a larger international role, both grow. The missing factor is Mexico's own vulnerability to precisely the sort of revolutionary tendencies it is now encouraging in El Salvador and elsewhere The public mythology in Mexico places the nation above such stresses on grounds that the Mexican revolution has already taken place and has been "institutionalized" in its politics. But Mexico is a country of stark economic and social divisions and, perhaps more important, it is a country itself caught up in change. Conceivable, as some in Mexico say, the encouragement of revolution abroad will spare Mexico the affliction. Perhaps it will help to bring it about. The question seems only not to be starting to move toward the center of Mexican discussion.

Mr. Reagan can hardly want to open his presidency with a fight with Mexico Certainly he cannot stake out a policy of support of the Salvadoran government on the basis of an assessment of Mexico's potential vulnerability that Mexican leaders, publicly at least, do not share. Yet Mexico is the country that counts in the region.El Salvador, Nicaragua and the rest are the sideshows. The United States' anxiety about externally supported revolution and Mexico's dangerous indulgence of it represent approaches that the two countries need to talk out at length.