"RECIPROCITY," in international trade, is one of those ideas that seem self-evident at first glance. What could be fairer than being fair? It's when you look more closely that certain doubts begin to form. What, exactly, does reciprocity mean? A tooth for a tooth? A car for a car? A jetliner for a jetliner? Under that strict rule, this country would shortly be importing very few automobiles, since it exports very few. But--and here's the catch--since this country imports few aircraft it would also be exporting fewer, with resulting layoffs in the aircraft factories.

That, presumably, wasn't quite what U.S. Special Trade Representative William E. Brock had in mind when he used the term in an international meeting yesterday. He intended to suggest that this country's barriers to foreign goods should be the same as its trading partners'. You don't have to be terribly perceptive to see that it's Japan that Mr. Brock has in mind. But it's hard to see how a legal rule of reciprocity would work there. True, Japan sells much more to the United States than it buys here. It's also true that the United States sells more --a lot more--to the Europeans than it buys from them. Country-by-country trade never balances. What about the worldwide trade totals? You probably are aware that the United States sells less to foreign countries, in total, than it buys from them. But there is also a huge flow of money into this country from American companies' foreign investments. It balances the trade deficit, and more. If this country tried to make its exports equal its imports, at a time when some $40 billion a year in foreign investment income is rolling in, the value of the dollar would shoot up and immediately frustrate the whole idea.

Mr. Brock knows all that. What he and the administration are most worried about is, apparently, one special case--Japan, and not Japanese goods in general but goods in one very special area, semiconductors and other high-technology electronic components. They fear that the Japanese manufacturers will achieve unfair advantages in this crucial industry, through restrictions on their own home market compared with open access to the United States. That is a legitimate concern for a government but, like all trade issues, it ought to be addressed directly and specifically in negotiation, rather than through rubbery and ambiguous terms like reciprocity.

Since there can be no clear and objective definition of reciprocity in world trade, it would inevitably be administered in response to the political impulses and necessities of the moment. If Congress were ever unwise enough to put this word into legislation, it would become a mere weapon for holding down all sorts of imports, and protecting American producers at a huge cost to consumers. That isn't reciprocity. It's protectionism.