Janus - the two-headed god who looked in opposite directions and gave his name to this coming month - presents a peculiarly fit image for describing the passage from 1977 to 1978. To a rare degree, the year now beginning, and a few after it, have already been mortgaged to the year now ending and a few before it.

To understand the heavy hold of the immediate past on the immediate future, it is first necessary to revise the judgments rendered by those of us who happened to be on the winning side in the long national debates about Vietnam and Watergate. The drives to end the war and impeach the president - while certainly necessary and important - were last gasps of the liberating impulse that dominated American politics, manners and culture in the 1960s.

They were not a new beginning. They were - as the sad example of the increasingly narrow work done by those heroes of Vietnam and Watergate, the investigative reporters, indicates - a dead end.

An event of far more enduring weight was the recession-cum-inflation of 1974-1975. Ten million people were out of work in this country, and millions more in other parts of the industrialized world. Businesses large and small were shaken if not toppled. Practically everybody suffered a drop in real income. The value of savings and pensions were diminished on a dizzying scale.

The effect was not only the widely publicized drop in business confidence. Far more important and less easy to describe was the impact on those who comprise the economic masses. Buffeted by a gale that demolished the expectation of endless improvement, they groped for moorings in economic folklore, a return to basics. But the economic folklore of the industrialized world is a hundred years out of date. It emphasizes individualism, and the virtues of thrift and hard work. It allows no scope to government for economic stimulation, for creating jobs or for restraining wages and prices.

The consequence has been a self-verifying cycle of political and economic frustration. Business investment has been too weak to kick off strong recovery. Popular prejudice has constrained government efforts to induce economic expansion. The same forces absolutely prohibit government restraints on wages and prices that would prevent stimulus from starting up inflation. With theinflationary threat thus kept alive, lack of business confidence is validated.

As a result, in almost all countries, governments and their leaders ran into trouble and maintained control mainly because their oppositions were also weakened. The case of President Carter seems to me especially instructive. He came to office pledged to stimulate the economy and hold down inflation by "direct" action on prices and wages. His stimulus package was truncated by popular dislike of a proposed tax rebate that had to be withdrawn. Business and labor opposition to "controls" killed all moves to hold down wage and price hikes.

Thanks to the do-it-the-hard-way folklore, a defect in the Social Security system - which could have been cheaply mended by tapping general revenues - had to be fixed by a tax increase that will cost a quarter of a trillion dollars over the next 10 years. That grossly excessive hike threatens the energy program, which also includes tax increases.

Politically, 1978 has to start with a quick tax cut - a tax cut void of serious reform. The absence of serious reform means an absence of serious effort to restrain prices and wages. So the country is still stuck in what that foremost modern economist Arthur Okun has called the "great stagflation swamp."

Largely because of huge harvests, the United States lucked out last year. Inflation and unemployment, while staying high, dropped slightly and there was reasonably good economic growth. But what struck me as evolution away from foolishly overoptimistic goals, struck people who started with a much higher estimate of Jimmy Carter as weakness and vacillation. So he has become to Washington and to the business community an object of scorn, and to the country at large a big yawn.

In these conditions - with a strong anti-inflation policy ruled out, and with consumer buying due to drop - it will take a lot of luck to do better in the near future. As between 1977 and 1978, my impression is that the best that can be expected here and abroad is more of the same. Thereafter, I fear, things are apt to go very sour.