HOW SERIOUSLY should you take the collision between France and the United States over the next round of trade negotiations? Answer: not very seriously. Trade talks are important because progress toward freer trade will begin to undo itself: rolling backward downhill under its own weight if it can't be kept moving forward. But the dispute at the Bonn summit meeting represents only the midpoint in a long negotiation.

It's not a quarrel over whether to talk -- everybody's in favor of that -- but only over whether to agree now on a date next year to begin talking. The Reagan administration has its own good reasons for wanting a firm commitment on a date. It is under pressure from American export industries, most especially the farmers, to show activities on their behalf. The coming conference would be very useful in deflecting the protectionist atrocities that are likely to spring up in Congress this year.

But Francois Mitterrand, the president of France, has other concerns, the first of which is the French parliamentary election a year from now. Mr. Mitterrand knows that the Americans, in any trade talks, are going to go very aggressively after Europe's protected agricultural markets with their high costs and their huge subsidies. He sees no need to give his conservative opponents the kind of electoral bonanza that would result from any hint of European concession to American farmers. That's a reason for delay that Americans ought to be able to understand. After all, the Reagan administration deferred serious discussion of nearly every international issue for the 18 months preceding November 1984.

Mr. Mitterrand is also trying, perhaps as a gesture to his own Socialist Party, to make himself the spokesman of the Third World in the councils of the industrial powers. The not-so-rich countries want to know exactly what the United States has in mind regarding these trade talks. They have an uneasy feeling that the United States wants to organize a worldwide push in favor of wider trade in the things that it exports, while citing political necessity to keep its own markets closed to all but the narrowest trickle of South American steel, Asian textiles and Caribbean sugar. You cannot say that these Third World suspicions are entirely baseless.

Mr. Mitterrand would like to know more precisely what the United States wants to discuss at this future conference and what it wants to achieve there. A good many other people, neither protectionist nor French, have been saying for some weeks that it would be unwise to proceed toward talks without a clearer understanding of the agenda. The Americans want the talks set first, to force agreement on the agenda. Mr. Mitterrand is saying that he does not care to be pushed. There will be trade talks -- but he wants to see the agenda established first.

This brief dispute is cause for hope because it's a sign of movement. It's the first of these economic summit meetings in three years to which the Americans have brought anything more than bland self-satisfaction with the alleged triumphs of Mr. Reagan's strategy. It's the recent hints of trouble ahead that have sharpened the administration's attention.

Economic policy is widely regarded as a rather boring subject -- at least as long as prosperity holds up and things are going as well as they are now in the seven big industrial democracies. The job facing those seven governments is to keep the subject safely dull, and to keep people yawning as they did at Bonn. That's never so easy as it looks.