Any argument for cutting Social Security benefits has to start out like this:

No, the goverment will not reduce the Social Security check of anyone currently receiving benefits. No one -- Democrat or Republican--says that anyone's checks will have to shrink. If you've been told that such a thing is apt to happen, you've been told wrong.

But many people -- Democrat and Republican -- think that the growth in future benefits should be slowed down. If they aren't, spending on the elderly will overwhelm the federal budget at the expense of everything else.

Here is one key fact that will be new to many people:

Social Security and Medicare are already the single largest expense in the federal budget. Projected outlays come to $225 billion in fiscal 1983, which is 30 percent of total federal spending. National defense is the second-largest spending category -- $221 billion, or 29 percent of the total. Third comes net interest on the national debt -- $96 billion, for 13 percent of the total.

All together, those three items take up 72 percent of all the money spent by the federal goverment. Every other function of goverment -- farm programs, job training, food stamps, national parks, administrative activities, economic development, public works, everything -- falls into the final 28 percent of the budget.

One proposed remedy for the coming Social Security deficit is to finance it with present income taxes instead of higher Social Security taxes. That's obviously nonsense. The goverment is not collecting enough income taxes to cover its present expenses. That's why there's a budget deficit.

If some income-tax receipts are now diverted into Social Security, the huge budget deficit will grow even larger. Or, Social Security beneficiaries will get an even greater portion of federal revenues while everyone else's programs are muscled out.

It is not immoral or illegal to suggest that Social Security must be changed. It is not a contract. It is a highly prized program of social-welfare benefits whose levels of payments are subject to political negotiation.

Some Social Security benefits have already been reduced. President Carter limited maximum payments for Social Security disability in 1980. President Reagan eliminated student benefits, survivor benefits for parents of children aged 16 and 17, the $122 minimum benefit for new retirees and the $255 lump-sum death benefit where there's no surviving spouse or dependent children. he current fight is over which additional benefits should be curbed, and who should pay for those that remain.

Present Social Security retirees (and their employers) paid far less into the system over the years than they are now taking out in benefits. As recently as 1970, the maximum tax for an employe was $569 a year; by contrast, maximum benefits are currently running at $729 a month for a single 65-year-old, and $1,093 for a married couple. For the present generation of retirees, Social Security is a stupendous windfall.

But what about today's young workers? Will they ever get their money's worth out of Social Security?

A recent study by the Federal Reserve Bank of New York says they will. Under present law, even a 21-year-old can expect to receive more in benefits than the total of his own contribution, his employer's contribution and the interest that money could be expected to earn. But young people will receive only a small windfall from the system, compared with the huge windfall received by their elders. That's an important fact to take into consideration in deciding who should pay for the Social Security deficit.

Today's retirees and near-retirees want younger people to pay -- by raising Social Security taxes, bringing federal employes into the system and imposing a later retirement age at the end of the century.

Younger people, on the other hand, might think it fair for the present generation of retirees to pay a share of the cost -- by accepting lower cost-of-living increases and a prompt change in the early-retirement age.

If Social Security benefits were taxed in excess of your own contributions, just as private pensions are taxed, both generations would pay. But the cost would fall only those with incomes high enough to be taxed.

A combination of solutions will be needed to keep Social Security afloat. Taxing income from Social Security should be number one on the list.