If we are to understand the depressing gabble that comes from our nation's capital, it is best to keep in mind a discovery made by Malcolm Muggeridge on June 12, 1948, to wit: at the zoo, crowds stare as intently into cages that are vacant as they do into cages that are occupied. I certainly hope that our debonair president will keep this in mind as he hears the whines and shrieks now issuing from Washington's giants.

With the crowds that go to the zoo, the image of the zoo is everything. For them, it is the idea of seeing that matters, and it matters far more than the actuality of seeing. Leave the cages empty with neatly printed signs describing the peculiarities of the animals that are not there and the crowds will stop and ponder the cages nonetheless. Window shoppers do the same on Fifth Avenue. The more renowned the establishment, the more intently the crowds stare, whether the windows abound with opulence or a naked mannequin.

The habit of mind that seizes Washington's sages is not much different. It is the idea of solemnizing that matters, whether there is actually anything to solemnize or not. Say something solemn and conspicuously asinine and you will be marked down as a man of moment there. Say something suave and true, and people shake their heads and groan.

Last week there was a lot of solemn pish posh heard in Washington about how President Reagan had lost his bearings. Well, I saw few signs of it; certainly there was no sign of it at his press conference on Thursday. There was also grim talk that his economic program was in trouble. Once again, I looked for the corpus delicti and could find only a jumpy stock market. It all brings to mind the astonishing blah that we heard last summer about Reagan the Genius, and the blah we heard the summer before last about Reagan the Simpleton, and the summer before that about Reagan the Has-Been.

Last week such venerable political observers as Joseph Kraft and Arthur M. Schlesinger Jr. droned on about how Reagan's supply-side economics was rooted in "the theory of expectations." That the stock market was in a state of swoon was supposed to show that Reagan and his supply-siders had already been unhorsed. Actually, Reagan's supply-side economics is not rooted in "the theory of expectations" at all. Where the great ones got this idea is a mystery. Possibly they are confusing the supply-siders with conservative economists from the University of Chicago and the University of Minnesota, who have espoused a "theory of rational expectations." Possibly they just have not paid much attention to the development of conservative economics since the Hoover administration.

At any rate, supply-side economics is rooted not in any fanciful theory of expectations, but in something sturdier, people's existing motivations. Expectations ebb and flow. Motivations are durable: they are the structure of what we call human nature. "Supply-siders," says Irving Kristol, a leading exponent of supply-side economics, "believe in allowing people to do what they clearly want to do: work, save and improve their conditions." Hence the president called for a tax cut.

What he got was a smaller cut than he wanted, and it will not have its impact until later than he wanted. Washington's dirge about the failure of the president's policy comes from those who have been peering into an empty cage. Only last week, with the beginning of the fiscal year, did the beast arrive, and it will be several months at least before one can expect effects. All we can say for now is that the Reagan policy appeals to our sense of logic, and it is hugely more prudent than the policies that brought us the highest peacetime inflation in our history, high unemployment and stagnant productivity.

The depressing gabble that issued from Washington's sages last week was unjustified. Inflation has already dropped dramatically, and our standing abroad is rising. Doubtless the administration is going to make its mistakes and take some lumps. It could use some breaks: congressional approval of AWACS for the good of its Middle East policy, congressional approval of new budget cuts, and above all patience--it takes time to make history.