The French, who politicize everything from industry to poetry, suddenly are having a lot of backaches, and socialism may be to blame. Polls show that since 1981 the percentage of people who say they are increasingly irritable has risen from 26 to 42 percent, the prevalence of backaches has increased substantially, as has the number of people who say they expect to be in an automobile accident. A decline in the nation's sense of well-being has coincided with the floundering of the first Socialist government under the Fifth Republic.

Four years ago this month, France elected Francois Mitterrand president and embarked on a historic journey in Europe's experience, a journey to the end of the line for socialism. Mitterrand, who once criticized Sweden's socialists for not dealing capialism "a mortal blow," has dealt one to socialism.

In the 1981 campaign, he promised stimulative spending through expanded entitlements, a shorter work week with no cut in pay, job creation through public-works spending, steeper taxes on the wealthy, sweeping nationalization of many industries and banks, and an attack on the independence of Catholic schools. Then he rounded on the country and began keeping his promises.

Because France listens intently to intellectuals, it often is late learning the obvious. It has now learned that much of the money pumped into the pockets of people who are working less was spent on imports. So while productivity fell, demand rose and the franc fell, unemployment rose, the purchasing power of the employed fell, and so did Mitterrand in the polls to a point lower than any postwar leader.

His biggest blunder was a bill to break Catholic schools to the state's saddle. The bill embodied traditional socialist thinking about conquering reactionary tendencies by nationalizing consciousness and liberating children from the dead hand of the past, including parents. This brought a million protesters into Paris streets in the largest demonstration since Liberation.

Mitterrand promised radicalism, and in two years he has delivered perhaps the most radical reversal in recent European history, imposing budgetary austerity more severe than Reagan has proposed. Candidate Mitterrand exhorted, "Let's break with the logic of profitability," and under his policies many businesses, alas, did. President Mitterrand now says, "We can no longer keep on crushing those who create wealth in France with taxes and social contributions."

He insists his conversion to tax cuts and spending restraints is just a "parenthesis" in the story of French socialism. But the parenthesis was preceded by failure so complete that the intellectual cupboard is barren of ideas about what can come next that will resemble socialism.

His semi-Reaganism has been called "supply- side socialism," but that is an oxymoron. The crux of supply-side economics is tax cutting to lighten the weight of the state on entrepreneurship, and the essence of socialism is the use of the state as the central directing source of entrepreneurial energy. Mitterrand lamely says that socialism just means a "mixed economy." If so, France has had socialism since 1945. Mitterrand's predecessor, Val,ery Giscard d'Estaing, warned that when the combination of taxes and social security contributions exceeds 40 percent, "we're in socialism." If so, socialism arrived under Giscard, when the combination neared 43 percent.

France has more politics than probably is good for it. With 468,000 municipal councillors, 2 percent of the electorate is in elective office. But what unquestionably is good about French politics today is that the political label with cachet is "liberalism." Here liberalism retains its traditional connotations, which are unlike those of American liberalism. Here it means an anti- statist emphasis on personal initiative and market mechanisms.

Mitterrand's personal initiative has been a hyperactive foreign policy, searching for what some European politicians, envious of Margaret Thatcher, call a "Falklands effect." But the principal effect of foreign events during the last decade has been to speed the collapse of confidence among French intellectuals on the left, who have abominable records of communist affiliation or fellow traveling.

Only intellectuals inordinately fond of abstractions (like the three great incompatibles, "Liberty, Equality, Fraternity") would take until 1975 to learn (from a book, of course: Solzhenitsyn's, which had its strongest impact here) about the concrete reality of the Gulag. Also instructive were Vietnam boat people, Cambodian genocide, Afghanistan atrocities and the suppression of Poland, a nation about which France has long been sentimental. Today French intellectuals, writing with the vigor of people making up for lost time, are robustly anticommunist on their flight toward the right.

One fine day in 1953, the front page of the communist newspaper L'Humanit,e had a black border and this headline: "Dreadful News: Stalin Is Dead." One fine day a headline may say: "Socialism too."