IS A NEGOTIATED solution possible in El Salvador? Each of us is entitled to a theoretical answer based on his hopes or fears as long as everyone realizes that the only answer that counts is one arising from an actual effort to find out. Therein lies the significance of the first reported meeting between Richard Stone, the president's envoy, and Ruben Zamora, a politician of the Salvadoran left. The United States means to draw the interested parts of the left into early elections run by the Salvadoran government. The left invites the United States to another sort of political process in which the rival armed forces are sorted out first. So wide is the gap that there is not the slightest chance it can be narrowed--if at all--without broad-ranging talks, of which the weekend encounter in Bogota was presumably a first installment.

It is the season for negotiating proposals in Central America. Nicaragua, for instance, calls for stopping the flow of arms to both government and guerrillas in El Salvador but to only one side (guess which) in Nicaragua. Fidel Castro says he would halt Cuba's dispatch of arms and advisers to Nicaragua if no arms and advisers went to anybody else in Central America. There is an airy quality to these proposals, which address large goals and come unattached to practical procedures to reach them. Nonetheless, Nicaragua and Cuba both now accept the collective nature of the region's security problems. The Contadora group has got to do more to help the parties get down to brass tacks.

The question of trust is basic in any negotiation, and the forces of the Salvadoran left, not to speak of the Nicaraguans and Cubans, have a way to go to reassure the Reagan administration and even many of its American critics.

The administration also has a way to go. It is not yet possible to say, for instance, whether Ambassador Stone's ultimate instructions will not be in effect to set the stage for American military action by showing that negotiations won't work. The moves that the administration credits with inducing fresh moderation on the Central American left sharpen the question of whether they are tied to precise, reasonable and achievable negotiating objectives, which remain-- publicly at least--unarticulated. The House vote to cut off the CIA's role in Nicaragua reflected the doubts Americans have about Mr. Reagan's policy, specifically, whether and how he will convert his building military credibility into negotiating coin.