Jean-Paul Sartre, 74, the French philosopher, novelist and playwright and the leading exponent of existentialism, died yesterday at a hospital in Paris. He had been hospitalized recently for treatment of a lung tumor.

The author of more than 20 plays, novels and philosophical treatises, Sartre once refused the Nobel Prize for literature. He also was an active member of the French resistance during the Nazi occupation of France and one of the world's best-known Communists -- although he never joined the party and often was harshly critical of communist governments

In his final years.Sartre became a virtual recluse in his high-rise apartment in the Montparnasse section of Paris, but he acted as a sort of godfather to small, radical student revolutionary movements and sometimes served as titular editor of their newspapers.

Apart from his writings and pronouncements. Sartre was widely known for his liaison with the feminist novelist and philosopher Simone de Beauvoir -- an association without marriage in a framework of mutual freedom that began in his student days at the University of Paris and continued for the rest of his life.

He stopped writing in 1975 because of failing eyesight leaving unfinished the last volume of a four-volume biography of Flaubert on which he had been working since 1960. "I walk a bit," he told an interviewer in the year he stopped. "I have the newspapers read to me, I listen to the radio, I try to make out what's happening on the television screen -- all time-killing activities. The sole object of my life was to write."

In the 1950s and early 1960s, his existentialism (often only partly digested) had been one of the prime sources of cocktail-party chit-chat and he became a sort of tourist attraction for visitors to Paris, who would hunt through the Left Bank cafes hoping to catch a glimpse of him.

But Sartre passed beyond existentialism in his enthusiasm for Marxism, and eventually the mass readership also lost interest. Existentialism did not exactly disappear, but it was assimilated into the ideals of "freedom," "authencity" and "doing your own thing" that became slogans for the student generation of the '60s.

Sartre's fame probably reached its peak in 1964, when he was offered the Nobel Prize for literature and turned it down. He explained at that time that he thought a writer "should refuse to allow himself to be transformed into an institution."

If not an institution, however, he was by then something like a monument: the man who had (not first but most spectacularly) challenged the theory of "essences" -- the idea, for example, that humanity is the essence of man -- which had been a dominant influence in philosophy, particularly Christian philosophy, since the time of Plato.

Dissatisfied by the "lucid blindness" of that system, in which he felt that the names of things were treated as more real than the things themselves, he worked out the deceptively simple-sounding thesis that "existence" precedes "essence," which might be translated into everyday language with a summary like: "You are what you do."

In one sense, existentialism is an effort to find the meaning of things in the things themselves rather than in any outside, preexisting system. There is not great problem in finding the meaning of man-made objects, since the meaning is established by those who make them and use them: a hammer means a device to drive nails. But as an atheist (coming out of a family divided between Lutheranism and Catholicism), Sartre could not accept the answer to the meaning of human life that had satisfied his ancestors: that man had a maker who gave life its meaning.

Drawing heavily on the work of earlier philosophers such as Soren Kierkegaard, Demund Husserl and Martin Heidegger, Sartre concluded, in effect, that each person is his own creator, that the individual creates his own identity and the meaning of his life through his own free and conscious decisions. The fact that this perception now seems rather banal shows how thoroughly Sartre's thought has permeated modern awareness, though nobody talks much about "existentialism" at cocktail parties any more.

Sartre's system emphasizes strongly two elements related to human decisions: freedom and responsibility, both of which impose heavy burdens on the decision-maker. He concedes that decisions are made in the context of a "situation," but the situation can be changed by a decision and therefore becomes a part of the decision-maker's responsiblity. A basic goal in decision-making is "authenticity," truth to the self with all its weakness and ambivalence, rather than "bad faith," which is a form of self-deception, accepting the role society imposes on one's self and thus becoming an object rather than a person.

These ideas seem to contrast sharply with the Marxism which was Sartre's second intellectual love -- a philosophy which sees the individual as almost powerless for self-determination, an object conditioned by large, impersonal social and economic forces that almost eliminate freedom and responsibility.

There has been considerable discussion (in which Sartre declined to take an active role) of the question whether he rejected existentialism in his later works or merely assimilated it into his brand of Marxism. An ambiguous piece of evidence is his 1960 "Critique of Dialectical Reason," in which he called existentialism "a parasitical system" and Marxism "the philosophy of our time." But he also warned that Marxism must be adaptable to particular situations, not ossified into an abstract, a priori system.

In practical terms, the question become almost meaningless. To most people who recognize the word, existentialism is not a complex, well-ordered body of though, but a set of attitudes and a vocabulary for expressing and developing those attitudes. And Sarte's Marxsm, in public actions as opposed to theoretical writings, seemed to be attached more to the process of revolution (a very existential activity) than to the philosophy of dialectical materialism.

In his later years, Sartre issued public denunciations of Communist governments (mostly the Soviet Union, but also Mao and Castro and even the city of Bologna) even more frequently than his denunciations of France and the United States. And in the end, his philosophy was absorbed more by capitalist than by Marxist societies, and Christian philosophers even developed their own brand of religious existentialism (true to the devout, anguished spirit of Kierkegaard, who was the system's true founder).

Sartre's philosophy owed much of its flavor to his childhood experiences, as he indicated in his autobiography, "The Words," which never gets much beyond the first dozen years of his life but treats that period with almost total recall.

Born in Paris on June 21, 1905, he was less than a year old when his father, a French naval officer, died. The result, he believed, was that "I have no superego."

This diagnosis has a ring of truth. Raised by a doting, widowed mother and a grandfather (Charles Schweitzer, uncle of Albert Schweitzer) who indulged and idealized him, Sartre recalled, "I was not taught obedience." But he did think he was taught role-playing, and his childhood experience may have convinced him of his basic principle that people are likely to become what they pretend to be. Sartre pretended to be the boy genius his grandfather expected. He recalled learning to read at age three of four by pretending to read -- and, five or six years later, becoming a writer by pretending to be a writer, through plagiarism.

Educated at first by private tutors and later in a public lycee (approximately equivalent to high school), Sartre entered the University of Paris in 1924.

After receiving his degree in philosophy i929, he served his two years of compulsory military duty (as a meteorologist) and then began a career as a teacher in a lycee in Le Havre. In 1933, a key year, he went to Berlin on a fellowship to study at the Institut Francais. There, he began to study the existentialism of Heidegger and the phenomenology of Husserl which provided the basic tools for working out his own system.

During the 1930s, he began to publish philosophical works such as "The Transcendence of the Ego." "Imagination," and "The Emotions," but the first book that brought him fame was a novel, "Nausea" (1938) which embodied many of his ideas in their early forms.

This set a pattern which was repeated often in the '40s and '50s. His ideas were given much more currency through fiction such as "The Wall" and "The Roads to Freedom," or plays such as "Dirty Hands," "No Exit" and "The Respectful Prostitute," than through such philosophical works as "Being and Nothingness" or "Existentialism and Humanism."

His close association with communism dates from his underground activities during World War II. After serving briefly in the army and a short period as a prisoner of war, he spent the Nazi occupation in Paris, teaching, writing and working with the resistance. One of his plays produced during the Nazi occupation was "The Flies," an adaptation of the Greek legend of Orestes with subtle anti-Nazi overtones added.

His anti-Nazism later developed into anti-Stalinism. He visited the Soviet Union twice, in 1954 and 1962, but he also strongly denounced the Soviet government several times -- notably in 1956 after the invasion of Hungary, and in the 1960s for the jailing of writers Yuli Daniel and Andrei Sinyavsky. An even more serious break came in 1968, after the quelling of the student uprising in Paris and the brutal suppression of the Dubcek regime in Czechoslovakia.

After that, his allegiance was given primarily to young revolutionaries, and his name appeared most often in small, radical periodicals with names like "Liberation" and "La Cause du Peuple," which would be distributed in only a few thousand copies. After he became unable to write, he would give occasional taped interviews to such periodicals, although he felt less comfortable in this form, and several books have been made from these interviews.

In one of his final statements -- an interview published in mid-March in Le Nouvel Observateur -- he said that his basic attitude remains one of hope. Asked whether he considered his work a failure, he gave a reply that might be his epitaph:

"I am not Shakespeare and I am not Hegel, but I have produced my works with as much care as I could. Some have been failures, surely: others less so, and others have succeeded. That is enough."