Sartre: A Philosophical Biography. By Thomas R. Flynn. Cambridge University Press; 436 pages; $39.95 and £30.

WHEN the French thinker and writer Jean-Paul Sartre died in April 1980, 50,000 people followed his hearse through Paris. It was a fitting tribute in a country where intellectual life is prized. Philosophers, though, are judged by their arguments, not their funerals. On that sterner test, how has Sartre’s philosophy held up? Thomas Flynn’s thorough new study offers expert guidance.

Most technical philosophers tend to look at the world as armchair scientists. They puzzle about time or knowledge, matter, numbers and chance. They ask how such things really are. Sartre, who also wrote bestselling fiction and plays, thought about the world as an off-duty novelist. He asked what the world was like for people. They were not detached physicists or passive observers. They lived, aided or obstructed by a material world, which included their bodies. For good or ill, they were thrown into contact with others. Sartre’s concern, in a phrase, was what it was like to be human. The topic sounded unmanageable. But its core elements were familiar enough: the mind, human values and human freedom. Sartre linked them together in big loose equations.

The human mind was free, notably in its imaginative capacity to entertain possibilities and think of the world as different from how it was. People were free of religious or ethical authorities, so obliged to find their own values. They were free finally to define themselves or choose a form of life as they pleased, for there was no human nature—nothing essential, that is—to being human. Those three freedoms added up to “existentialism”, an otherwise obscure label by which Sartre’s thought became known. How far it impresses you will depend on whether you prefer philosophy in careful, dry bits or in bold flashes that briefly light up a territory.

Sartre’s most famous philosophical treatise, “Being and Nothingness” (1943), suffered from trying to embed pointed insights into human thought and experience in a dense metaphysical theory that split not just mind from world but the mind of any one person from everyone else’s. In compensation, the book displayed Sartre’s rare psychological acuity and brought out the pervasive role of the imagination.

Sartre stressed an ever-present emotional element in human thinking that philosophers preoccupied with truth and validity tended to overlook. He was fertile and original in putting previously neglected mental phenomena such as shame, pride and fear at the centre of how to understand self-awareness. To cultural and gender studies he bequeathed the enticing idea that the judgmental look or predatory gaze of others shaped and commonly distorted a person’s sense of who they were.

In a typical twist, Sartre added that people were entirely free to reject the verdicts of others and their stereotyping. To deny that freedom involved self-deception or bad faith. So, generally, did blaming your situation on your past, your parents, the unconscious, social pressures or human nature. Those were craven excuses. At any moment you could avow or disavow your situation. Strictly, it was yours only once you claimed ownership. Such freedom was daunting, Sartre recognised. In his brutal phrase, everyone was “condemned” to be free.

In the philosopher’s superabundance of ideas, imagination did most of the work. Neither passive nor whimsical, this power of supposition underlay the simplest intentions and grandest plans. Its uses ran from the banal and domestic to the heroic and political. As the power to suppose that society might be different, imagination dominated Sartre’s radical politics. On Paris walls in the student upheavals of May 1968 appeared the Sartrean slogan, “Power to the imagination”.

Besides a weakness for overgeneralisation, Sartre tended to over-empower the mind. At one extreme, he was never far from gifting people with mysterious mental powers to shape events. His runaway prose tended to blur the simple point that picturing a better society did not create a better society. Imagination was needed for action but was never itself enough. People still had to act.

At another extreme, Sartre risked lapsing into stoical banalities. People had no protection, he admitted, against time, pain or death. They were still free, the philosopher insisted, to say what those ills meant to them. People were free to grant or deny value to life’s vicissitudes. How far, though, did refusing “ownership” of pain remove pain?

Sartre’s published output is reckoned at 20 pages a day throughout his working life. No English-speaking philosopher has read that vast corpus with greater industry than Mr Flynn. His new biography scrutinises the works chronologically from start to finish. It includes Sartre’s fiction and plays, as well as the political or critical essays. Mr Flynn has done Sartrean initiates a large service, but this is not an introduction.

Though he writes within Sartre’s thought, using his vocabulary, Mr Flynn stresses unresolved puzzles. Technical philosophers can sound as if they are trying to describe a world without humans, Sartre as if describing human life without a world. Sartre saw the difficulty, but never completed his later attempts to relocate the isolated mind of “Being and Nothingness” in a social world of rights and duties. Nor did he let up on the clash between limitless freedoms to change attitudes to a situation and their apparent powerlessness to change the situation itself. A cheery, can-do sort would say that life is not always so bleak. People are rarely or completely so trapped. No, indeed. But sometimes they are, which is one reason why Sartre’s work, though little read these days, remains interesting and even topical.