When some Homer comes to write an epic poem about the intellectual odyssey of American conservatism, he will want to dwell on an episode in Sidney, Ohio, in autumn, 1984. There, President Reagan, custodian of conservatism, speaking from the rear platform of a train, said: "My opponent Mr. Mondale offers a future of pessimism, fear and limits. . . ."

It is enough to curl your hair: Mondale, that cad, was going around scaring the children with the thought that there are limits.

Time was when conservatism's proudest boast was a flinty realism. It looked life in the face without flinching from the facts about the costs of things. No more.

Mondale was somewhat shaky on the subject of limits. Promising more "compassion" for the poor, and for lots of other folks, and promising to trim the deficit at the same time, Mondale sounded like the will that Francois Rabelais left when he died in 1533: "I have nothing. I owe much. I leave the rest to the poor." But conservatives are supposed to be different.

Reagan's statement in Sidney talked about the Republican future of "hope, confidence and growth." The question today is about the third item: Will economic growth free Americans from "limits"?

Do not try to talk limitlessness to the Reagan aides who are aging rapidly as they toil at producing a budget. Their task is to do what Reagan wants done, within the limits (if you will pardon the word) he has decreed. We must back up in order to take a running jump into this subject.

In fiscal 1984, the year of the second Reagan landslide, federal revenues were almost exactly the share of GNP (18.7 percent) that they were in 1964 (18.4 percent), the year of the anti-Goldwater landslide. But federal outlays as a percentage of GNP have risen from 19.2 to 23.5. Reagan says that tax increases will not be part of his deficit-reduction plan. Do you want to know the plan? Are you sitting down?

For fiscal 1986, Social Security ($200 billion) and defense ($284 billion) are to make up more than half the budget. Mondale forced Reagan to pledge (actually, to admit) that Social Security is untouchable. And Reagan wants his defense requests saluted, not touched. Interest payments are not optional, and they are the fastest growing part of the budget. They are $154 billion. The Social Security, defense, interest total: $638 billion.

The nice thing about a trillion-dollar budget is that it simplifies some calculations: $638 billion is 63.8 percent of the budget, almost two-thirds.

Reagan says the deficit, which is 5 percent of GNP, must shrink to 4 percent in fiscal 1986, 3 percent in 1987 and 2 percent in 1988. To get there from here, non-defense and non-Social Security spending cuts must total $42 billion in fiscal 1986, $85 billion in 1987, $110 billion in 1988. This must come from a target area of about $300 billion, more than one- third of which is Medicare or Medicaid.

This program of savings must be put in place this coming summer. Obviously the lead must come from the Republican- controlled Senate. Next summer 40 percent of the Republican senators (22 of the 53) will already be preparing for their 1986 reelection campaigns, and will be in no mood to scorch the earth where social programs stand.

Furthermore, low-income support programs, which received a disproportionate share of the cuts voted in 1981, cannot be cut again. So, to achieve the deficit-to-GNP relationship Reagan stipulates, within the limits he has stipulated, will require a frontal attack on the government's discretionary spending, an attack incomparably more radical than anything attempted or even dreamed of in 1981. The list of programs that must be eliminated (in alphabetical order, beginning with all agricultural programs, and Amtrak, and running through flood control, student loans and much else) fills many typewritten pages.

Now, remember, the "safety net" cuts are done. Today's long list of programs on the block are middle-class programs -- programs Republicans support. So, suppose Reagan sends such a list to Congress for execution. If Tip O'Neill is cunning and bitter -- and he is both -- he will urge Democrats to abstain from voting. The result will be that Republicans swat the budget, like a shuttlecock, 16 blocks back down Pennsylvania Avenue.

If the programmatic consequences of Reagan's goals and limits are proposed to Congress or (as is more likely) leaked, there will be a political firestorm. The resulting cinders will have to be sifted through a fine sieve to find even a charred remnant of Reagan's influence on the budget process.