In politics, as in commerce, the best rule of thumb is "buyer, beware." We should be extremely suspicious of any governmental institutions that do not have trade-offs and hard choices wired right into them.

Candidates for office, and their all-gain, no-pain promises, are obvious targets for scrutiny. But they are not the only ones.There is, for example, the process of creating laws by voter initiative.

California made the initiative famous, but nearly half the states allow it in one form or another, and more may do so. The governor of Kansas, Democrat Joan Finney, was elected last year on a populist platform that stressed the right of the people to make their own laws. She describes enactment of an initiative law as a major goal of her four-year term.

Defenders of the initiative process like to portray critics as arrogant elitists who do not believe the American people are competent to make sensible decisions about important public policy. But the most important argument against initiatives has nothing to do with intelligence or elitism. It has to do with the absence of hard choices.

Voters who cast initiative ballots are allowed to mark in favor of massive environmental cleanup campaigns or guaranteed levels of educational spending without having to think about how to pay for them. They are allowed to support drastic reductions in state taxes without considering which of their services they would like to see taken away in consequence.

Given an extensive enough education campaign, of course, a pressure group that opposes a particular tax cut or spending initiative can make its consequences seem so horrendous as to kill it off.

Most of the time, that does not happen. The voters are essentially asked whether they would like to give themselves a Christmas present, and they say yes.

It is the job of the legislature, months and years afterward, to pay for it, as Massachusetts is paying now, with an enormous deficit, for the 1980 initiative that placed a ceiling on local property taxes and forced large increases in aid to localities.

In the same way, California is paying for Proposition 98, the law requiring a level of state education funding that the state's current revenues cannot reasonably support.

Then there is the judiciary. Most judges believe that they inhabit a world of hard choices, and in their way, they do. The judge who commands a state to spend more money on its prisons or mental institutions, or declares a school financing system inadequate, or orders the doubling of a local property tax, as happened in Kansas City, Mo., may genuinely agonize over the decision. He may see it as a last resort to protect the constitutional rights of prisoners, or mental patients, or schoolchildren.

But it is a one-dimensional decision, nonetheless. Judges don't have to choose between more money for prisons and more money for schools; they are perfectly free, if the right cases happen to come before them, to insist on both.

The concept of limits makes its appearance only when the legislature or the city council tries to cope with the cumulative impact of all its judicial mandates and obligations and finds it impossible to do so, given the reluctance of the voters to accept any sizable tax increases. Only then are we talking about a hard choice - a choice between worthy causes, in which somebody who deserves better has to lose.

The fiscal troubles causing hardship nationwide have been made worse by years of decisions and commitments by political actors who chose not to frame them as the hard choices they really were.

Most of us learned a long time ago in our private lives that we cannot have everything we want, at least not all at once. There is a reason "no free lunch" has become a cliche; the reason is that we all know it is true. It makes so much sense, in fact, that the sooner we start applying it to our public lives, the better off we all will be.