WASHINGTON - A useful maxim teaches us that nothing is to be gained by beating a dead horse, but the metaphor permits exceptions. The Budget Summit Agreement of Sept. 30 is as dead as the pharaohs, but a post-mortem may be useful all the same.The House performed a public service at 1:30 Friday morning. Members of both parties ganged up to kill a deal they had no part in making; they refused to swallow this particular fraud. It was a late hour but a fine one. In the subsequent showdown with the White House, the heavens did not fall after all.

Lessons may be learned from the summit process, from the aborted agreement itself and from the underlying reasons for this fiasco.

The process was indefensible. From time immemorial, Congress has worked through its standing committees. Heaven knows the committee system is subject to abuse, but most of the time the system works. It would have worked this year if the leadership had insisted on it working.

Instead, fewer than a score of men - a dozen from Congress, half a dozen from the White House - arrogated to themselves the responsibility for drawing up a kind of treaty to be imposed upon captive nations. They huddled in private at Andrews Air Force Base. At the last minute, under threats of governmental paralysis, they called for ratification. At a breakfast with senior correspondents Friday morning, Speaker Tom Foley acknowledged that the summit process had aroused "resentment." He put it mildly. The process fomented outright rebellion. So much for process.

The agreement itself suffered from inherent fraud. It purported to produce a reduction in the federal deficit of $500 billion over the next five fiscal years. In page after carefully tabulated page, the authors described how this could be accomplished. But this was transparent hokum. It is hard enough to predict income and outgo for even one fiscal year. A reliable two-year budget is made of moonbeams. To predict that mandatory outlays in 1995 will amount to precisely $703.8 billion is absurd.

The same spirit of solemn nonsense hovered over the economic assumptions. Gazing into a crystal ball, or looking intently at entrails, the summit conferees made bold to predict changes in the gross national product: In 1991, up by 1.3 percent; in 1992, up by 3.8; in 1993, up by 4.1; in 1994, falling off to 3.7; in 1995, retreating to an even more modest 3.5 percent. Says who?

Economists are notoriously off the mark - wildly off the mark - in making such predictions. No one knows whether 91-day Treasury bills will carry an interest rate of 4.2 percent in 1995. The rate is unknowable. Will unemployment rise to 6.4 percent in 1992? Will oil prices average $21.79 a barrel in 1993? Where did the 79 cents come from? Pfui!

Members were terrified of purported "cuts" in spending for Medicare. In point of fact, outlays for Medicare would not have been cut. These were reductions in projected spending increases. Actual spending would have climbed by at least $15 billion over the five-year period. Another provision claimed "savings" of $8.1 billion by eliminating a lump-sum option for civil service retirees. This too was the stuff of moonbeams. Over time there would be no savings at all.

Underlying the failure of Congress is a failure of will. Members of the House and Senate, taken as a body, simply are not serious about reducing the deficit.

The budget is larded with fat. It oozes fat. Given the awesome prospect of monstrous deficits, members ought to ask of every appropriation: Is this necessary? Is it absolutely necessary? Is it absolutely, positively, unavoidably necessary? Or is the proposal merely desirable? Can we do without it for a year or so?

Until the day comes when such questions are seriously addressed, we will stagger on from crisis to crisis. If a private business conducted its affairs as stupidly, the business would go broke. Year by year, that is where Congress is taking us now.