THANKS TO PRESIDENT Reagan's new budget, the fairness issue is back in the news. As the president's own pollster has told him, most Americans think his policies are tilted toward the rich at the expense of ordinary people.

The fairness issue may arouse the American electorate to action, but don't bet on it. Americans have always been ambivalent about fairness. As a culture we believe in competition, and in an unequal distribution of rewards. Voters don't get excited about closing tax loopholes for the rich, probably because many Americans live with the belief that soon they will be rich, and they want the tax loopholes to be in place when they get there.

But there is more to fairness than Reaganomics and tax loopholes. At some level we are talking about the essence of democracy -- genuinely equal opportunity. Our constant friend de Tocqueville noted l50 years ago that a government has the power to establish aristocracies or to create true democracy merely by regulating the laws of inheritance, which he said should "be placed at the head of all political institutions."

The laws of inheritance, of course, are taxes. De Tocqueville argued that "when framed in a particular manner, this law unites, draws together, vests property and power in a few hands; it causes an aristocracy . . . to spring out of the ground." Drawn another way, he continued, an inheritance tax shatters special privilege and "signals the coming of democracy."

The fairness issue becomes serious business if we go so far as to ignore de Tocqueville's wise warning and allow the government to nurture and protect an American aristocracy. And it may be coming to that. The American upper classes have grown considerably. Today nearly 6 million American families have incomes greater than $50,000. That is a lot of well-to-do people -- so many that a lot of them have obviously decided that their status is normal, even average. (Actually, $50,000 is more than twice the average family income.)

The upper classes often behave as though they are entitled to their grand style of life, and entitled to preserve it even if that requires government intervention on their behalf. Because the well-to-do are particularly effective at getting their way, there is now a great deal of government intervention on behalf of rich people.

One example: How many ordinary Americans realize how their wealthiest compatriots can evade paying income tax on money set aside for their comfortable old age, and how they can pass it on to subsequent generations without paying estate tax? Most people are probably oblivious to these provisions, which are of great interest only to wealthy people, and which are so complicated that they aren't easy to grasp. But the pension tax provisions are, in the words of one congressional staff expert, "the sweetest tax shelter going."

The pension law was written not just to shelter income, but to encourage savings, and to encourage firms to set up pension plans for their workers.

But the law's provisions are more efficient at sheltering income than at promoting pension plans for rank-and-file employes.

To try to understand this sweetest of all shelters, consider a highly paid professional like a psychiatrist who works by himself. The law says that he can take money out of his current income, without paying any income tax on it, to build a nest egg that will give him a retirement income of $90,000 a year beginning at age 62.

Say this psychiatrist decides at age 50 that it's time he began preparing for his retirement. By consulting a lawyer and an accountant, he would learn that under current actuarial assumptions (that is, assumptions about how long he is expected to live, and what rate of interest his money will earn), he would need a fund of $1.3 million on his 62d birthday to insure that his twilight years were cushioned with a $90,000-a-year retirement income. The law permits this doctor to make roughly equal annual contributions toward that goal over the next 12 years (in other words, from age 50 to 62).

Say the psychiatrist is now earning $165,000 a year. He can set aside $81,190 this year (if he is willing to give up this much income) to invest in his retirement fund, deducting that amount from his taxable income. If he could afford to do this, he'd pay tax on less than $84,000 in earnings for this year.

If it weren't for this tax shelter, this same money would provide about $40,000 to Uncle Sam (assuming it would be taxed at roughly 50 percent, and assuming the psychiatrist didn't find some other tax gimmick). In other words, the psychiatrist has "borrowed" $40,000 from Uncle Sam to invest toward his old age. And over the next 12 years, that borrowed $40,000 will earn another $31,800 in interest, assuming the 5 percent interest rate the law says must be used in calculating retirement plans of this kind.

(If the return on money invested in the plan goes above 5 percent, contributions to the plan must be adjusted accordingly to try to hit the original $1.3 million target for the pension fund. If the fund exceeds that amount at the time our doctor retires, the excess reverts to him as taxable income. In fact, though, the fund will go higher than $1.3 million 12 years from now, because beginning in 1986 the $90,000-a-year figure will be indexed to inflation.)

During the 12-year life of his new pension plan, our psychiatrist can make up to $200,000 or more by investing money that would have gone to the federal treasury were it not for the law that created this tax shelter. This is not what Huey Long had in mind when he said share the wealth, but it certainly is a way to share the earning power of money that only avoided the clutches of the Internal Revenue Service because of the federal tax law on pensions.

This hypothetical example is simpler than most real-life situations, of course. Most professionals, independent businessmen and corporate executives who can exploit this pension shelter will take more than 12 years to build up their fund for retirement, so would make smaller annual contributions. And most of them will have partners or employes who must also be covered in such a pension scheme, making it more complex. But the basic facts recur in every case. An individual who earns at least $90,000 a year is entitled to the $90,000-a-year pension, and entitled to avoid paying taxes on that portion of his earnings that is contributed to the pension.

But that isn't all. We're talking here about old age. Imagine, God forbid, that our psychiatrist dies at age 65, when his retirement fund -- built up with untaxed income -- is still about $1.2 million. Imagine further that he has a wife and two children. Now watch what he can do with his estate.

Under the latest tax laws (all passed during the Reagan presidency), the psychiatrist is permitted to leave all his money to his wife tax-free -- she'd have no estate tax to worry about. But there are other attractive options.

The law provides that $100,000 from a qualified pension fund can be left to anyone the deceased has chosen, tax-free. And the 1981 tax law provides that (after 1987, when the act comes fully into effect) $600,000 can be left to anyone else, without paying estate tax. Hypothetically, our psychiatrist could leave $700,000 to his children and grandchildren, on which they would pay no estate tax. He could leave $500,000 directly to his wife, on which she would pay no estate tax.

(Federal estate tax now can claim more than half of a taxable inheritance, though the top rate will decline to 50 percent under a 1981 law.)

If his wife lived out the rest of her life on the income from her half-million, she too would be entitled to leave up to $600,000 tax free to whomever she chose. Her full estate could pass untaxed to her children or grandchildren.

So in the end, the money our psychiatrist sheltered from tax to build up his pension fund would go not to his comfortable old age, but to easing the lives of his heirs and assigns, who would use it long after he was gone. Assuming the children and grandchildren were in a lower tax bracket than the psychiatrist had been, they would pay a fraction of the tax he would have paid to the government in the first place had this money been subject to ordinary income tax, like the earnings of most Americans.

(If our psychiatrist did not die, but lived to the age of 80 collecting his $90,000 annually, he would have to pay income tax on that money every year.)

The pension and inheritance laws can help upper-income Americans to pass on an exalted economic status to their children by exploiting a tax gimmick that simply is not available to the overwhelming majority of their countrymen. By such means can the American upper classes practice their version of equal opportunity.

"The system" has always been skewered against poor people -- this is the American way, and will remain so. The interesting political question is whether the new facts of life in America -- low or no economic growth, sharply reduced opportunities for upward mobility and resulting restiveness in the middle classes -- will produce a new public attitude toward special government protection of private wealth.

Incidentally, the law that provides for a $90,000-a-year pension was actually reformed quite radically just last year. Until passage of the 1982 tax bill, rich Americans were entitled to pensions financed by tax-sheltered contributions of $136,425 a year. However, in 1981, congress also changed the inheritance tax law, substantially reducing that levy.

De Tocqueville said that "when the legislator has once regulated the law of inheritance, he may rest from his labor." It isn't time for a rest yet.