WHAT, AFTER ALL, is ''the Boland Amendment''? Mr. Reagan's critics insist that it is a law, by which they mean that it is a hard and fast, specific and comprehensive statute that he or others violated and for which they, and perhaps also he, must somehow pay -- by political disgrace or by being held to account on derivative perjury or conspiracy charges or conceivably in the president's case by impeachment. The president's defenders hold that the Boland Amendment in its several versions is basically a congressional policy statement that advises the president and limits some of his options but does not remove his capacity to conduct policy and certainly does not remove his capacity to conduct policy by means not explicitly outlawed.

This is the broad context in which so many interested parties to and observers of the Iran-contra hearings have come to sound like lawyers -- in fact, like constitutional lawyers. Does the Boland Amendment cover simply the activities of executive departments or also the activities of the president's National Security Council staff? it is asked. Does it ban not only the use for the contras of appropriated funds but also the use of private and foreign funds that Ronald Reagan, Oliver North and others scrounged up when appropriated funds were proscribed? Does its power of the purse give Congress the upper hand? Or is the president liberated by the countervailing doctrine of separation of powers and by his constitutionally endowed power to conduct foreign relations?

It may be useful to recall that this whole thing started as a policy argument and was being conducted, harshly but properly, in political channels by two sets of players -- anti-contra legislators and a pro-contra administration -- who felt equally strongly about it. But then Mr. Reagan rejected the rules and the outcome of the confrontation. Whether he acted legally or used more than the president's due share of constitutional authority is not so important here as that he acted secretly and deviously, in respect to Iran and also in respect to Nicaragua. He talked one policy and conducted another out of sight.

This performance leaves him poorly placed to complain about his critics' conduct now. No doubt there is a partisan aspect to the charges of illegality and overreach: an effort to build up steam for one final assault on presidential Nicaragua policy and a parallel effort to punish Mr. Reagan. But there is also an aspect of disinterested outrage to find that the president, having lost a political argument, did not simply accept the verdict or try as he appropriately could have to reverse it; he secretly circumvented it.

Whether, regarding the Boland Amendment, he or others broke the law or whether he abused his constitutional powers is something that will be determined -- or, as tends to happen in these fundamental confrontations, finessed -- in the weeks to come. But this is, we believe, what the odd and rather formless but necessary battle over the Boland Amendment is about