Not far from Berlin’s Brandenburg Gate lies the Holocaust Memorial, a vast grid of nearly 3,000 concrete blocks that span a field of 19,000 square meters and vary in height. Some rise only to the knees; others loom above the head as one descends the sloping plain to its center. The memorial was built only after a protracted debate as to whether such a sobering reminder of the darkest chapter in Germany’s past should stand at the heart of the nation’s newly refounded capital.

Following the memorial’s inauguration in May 2005, a reporter for the weekly Die Zeit took note of a solitary visitor, “a gentleman with snow-white hair” who was standing near an ice-cream van. “His hand is pensively holding his chin. He is looking at the people surging amid the stelae, the catch-me-if-you-can games of the pubescent, the photo-shooting fathers, exhausted pensioners. The man is standing there in silence.” He observes the whole scene “as if he were watching a sociological experiment.” But he has an air of dissatisfaction. “What is he thinking? ‘No comment.’” says the man. “He does not want to talk about it in public, not yet.” As the reporter leaves, the man’s “white hair can still be seen among the crowd.”

The pensive man with the snow-white hair was the philosopher and social theorist Jürgen Habermas, who for more than six decades has played the part of gadfly in modern Germany, just as Socrates did in ancient Athens. Even at his ripe age—he is now 87—Habermas’s passion remains undiminished. As a public intellectual, however, he may seem an unlikely hero. We live in an age when what some of us still fondly call “the public sphere” has grown thick with personalities who prefer the TED Talk to the printed word and the tweet to the rigors of rational argument. For Habermas, it’s clear that without the constant exercise of public deliberation, democracy will collapse, and this means that citizens must be ready to submit their arguments to the acid bath of rational criticism. The debates that preceded the construction of the Holocaust Memorial brought bitter memories to the surface—the novelist Martin Walser complained of “a monumentalization of our disgrace”—but for Habermas, a willingness to engage productively in self-criticism is a prerequisite for democratic consciousness. National pride in the conventional sense leaves him cold: In an essay for Die Zeit, he responded to Walser, emphasizing that “anyone who views Auschwitz as ‘our shame’ is more interested in the image others have of us than in the image German citizens retrospectively form of themselves in view of the breakdown of civilization, in order to be able to look each other in the face and show each other respect.” Habermas argues instead for “constitutional patriotism,” a sense of loyalty to the principles and procedures of the modern democratic state.

The ideal that most animates Habermas is a belief in the possibility of a genuinely critical and self-reflexive form of modern consciousness that can serve as the groundwork for politics. But for this very reason, he is a thinker who embraces complexity over dogma and has little interest in theatrical display. After a recent visit to a philosophy seminar in Munich, Habermas left students with the impression that he “is not a charismatic figure.” The students “never experienced any rhetorically incisive statements or any of the charming tolerance that bends over backwards to achieve compromise and accepts any statement, no matter how absurd, which is so common in the humanities today.” Nor did they observe any “posturing in the style of a grand master.” But they could still recognize the passion in his deliberation: “When Habermas thinks—and, at some point, he thought so intensely that he apologized for his ‘stuttering’—you always get the impression that he is, in fact, entering into an intellectual wrestling match with a problem.”

For the assessment of a philosopher’s legacy there can be no objective measure. But few would contest the verdict that Habermas has achieved—in both his philosophical work and in his role as a public intellectual—a place of enduring significance that surpasses that of any other thinker in our time. The definitive new biography by Stefan Müller-Doohm, first published in German two years ago and now available in an English translation by Daniel Steuer, lays out the evidence for this conclusion with great care and enormous sympathy for its protagonist.

The sympathy is justified. Since his earliest years as a philosopher and public critic, Habermas has served as a kind of moral compass, not only in Germany but across Western Europe. Even for those of us who have not had the privilege of working with him, his guidance has proved indispensable. It is, of course, a strange experience to read a biography of a still-living author. But we can only be thankful that he is still with us and has not yet grown tired of his task as defender of what he calls the “unfinished project of modernity.”

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Habermas was born in June of 1929, at a moment when the chances for democracy in modern Germany seemed uncertain. The Weimar Republic had achieved only a partial political equilibrium after the disastrous hyperinflation of the early ’20s, and compromise between the political factions on the left and right remained possible thanks chiefly to the “great coalition” of the center engineered by Gustav Stresemann, the conservative statesman who served as chancellor in 1923 and then as foreign minister until the fall of 1929. Stresemann’s death, nearly coincident with the stock market’s collapse, signaled the beginning of the end for German democracy. The centrist coalition began to unravel; the Social Democrats tried to sustain the government, despite the fact that the German president, the aged and decorated war hero Paul von Hindenburg, was opposed to the republic.

Ernst Habermas, Jürgen’s father, was the son of a Protestant parson and conservative in his political beliefs. In the spring of 1933, soon after Hitler came to power, Ernst joined the Nazi Party, and with the outbreak of war in 1939 volunteered for military service in the Wehrmacht. Jürgen was only 10 years old when he became a member of the Deutsches Jungvolk and later the Hitler Youth. But this was hardly an act of ideological conviction—membership at the time was compulsory—and Jürgen escaped paramilitary training by announcing his intention of becoming a physician. It is not irrelevant to note that Habermas was born with a cleft palate and in his early years endured a series of surgical procedures that left him (as he would himself observe) with a strong feeling for human fragility and interdependency. Medical textbooks at the time listed his own condition among the “hereditary diseases,” a fact that may have enhanced his resistance to Nazi indoctrination. In February 1945, the 15-year-old received notice to join the Wehrmacht for the final desperate battle against the armies of the invading Allies, but by a strange twist, Habermas was absent when the military police knocked at the door. Soon thereafter, the Americans arrived, and the war was over.

The German statesman Helmut Kohl, who is a year younger than Habermas, has spoken of the “blessing of late birth.” Both men belong to what the historian Dirk Moses has called the “forty-fivers,” the generation of German politicians and intellectuals too young to have seen active military service during the Third Reich but old enough to have known the horrors of the war. Many in this generation did not wish to dwell on uncomfortable questions about national responsibility. Defining 1945 as a Stunde null (or “zero hour”), they readily embraced the anticommunism that swept through Western Europe as an ideological warrant for the Marshall Plan, and in the following years adopted an attitude of cool pragmatism as West Germany underwent the three decades of recovery and transformation known as the “economic miracle.”

Konrad Adenauer, the first chancellor of the Federal Republic and the standard-bearer for the conservative Christian Democratic Union, campaigned under the slogan Keine Experimente! (“No experiments!”). For Habermas, however, the tyranny he’d witnessed as a child wasn’t a license for present conformity but instead a spur to political criticism. Refusing to forget what had happened in his own country, he became a tireless opponent of all conservative and nationalist values and a fierce champion of what is typically called Vergangenheitsbewältigung, or “working through the past.” Memories of the Hitler era may also explain Habermas’s personal indifference to charismatic authority. Even in the most challenging and technical aspects of his philosophy, one can sense a devotion to the ideal of a public sphere that draws its power from nothing else than rational debate. If there’s a single principle that animates his entire philosophy, it is that power alone is not justification: The only force that can be valid within a democratic system is what he calls “the unforced force of the better argument.” Sustaining this principle is what distinguishes democracy from tyranny.

Habermas came to this idea after several trials, and only after he had worked through his own attachments to the less rational strains in German intellectual history. In his uncle’s library, he found works by Kant and Nietzsche; he turned the basement kitchen into a private sanctum where he would loudly declaim lines from Thus Spake Zarathustra. But “in the end,” he later recalled, “the associations with the decrepit slogans of the Nazis…became too embarrassing.” By 1948, his studies had shifted from medicine to history, philosophy, German literature, and economics; he found himself especially captivated by “the opaque rustling” of existentialism.

After brief periods of study in Göttingen and Zurich, he moved in 1950 to the department of philosophy at the University of Bonn, where he struck up a friendship with Karl-Otto Apel, a somewhat older recent graduate who had worked under the philosopher Erich Rothacker. Rothacker’s entanglement with the Third Reich had been extensive—he’d even offered radio lectures for Goebbels’s Ministry of Propaganda—but at first his influence on the young Habermas was considerable. Then in his early 20s, Habermas focused on topics such as philosophical anthropology and Wilhelm von Humboldt’s philosophy of language. At the center of his studies, however, was Martin Heidegger. In academic essays and in newspaper pieces, he adopted Heidegger’s characteristic mannerisms, with allusions to “the concealment of entities” and “the emergence of the sign.” By early 1954, Habermas had concluded his doctorate with a dissertation on “the absolute and history” in the philosophy of Friedrich Schelling, the early-19th-century thinker whose legacy flowed into the darker tributaries of modern irrationalism.

In the summer of 1953, however, Habermas was confronted with the scandal of Heidegger’s political past. Apel gave his friend a copy of Heidegger’s An Introduction to Metaphysics, a book first published in 1935. In the new edition, a reference to “the inner truth and greatness” of National Socialism remained in place, without explanation and with only minor modifications. Habermas, outraged, responded with an essay published in the Frankfurter Allgemeine Zeitung. “The lecture of 1935,” he wrote, “mercilessly unmasks the fascist colouring of that time.” What troubled him most of all was Heidegger’s failure to offer any explanation after 18 years: Was the “planned murder of millions of human beings,” Habermas asked, a mere signpost along the “history of being”? Or was it not “the foremost duty of thoughtful people to clarify the accountable deeds of the past and keep the knowledge of them alive?” It was time, Habermas declared, “to think with Heidegger against Heidegger.” This moment of political reckoning brought his early phase of existential enthusiasm to a decisive end. For his biographer, however, the incident also marked a turning point in the young scholar’s career. Though Habermas typically avoided confrontation at home, Heidegger belonged to the same generation as his own father; his condemnation of Germany’s most celebrated philosopher took considerable courage. With this act, the 24-year-old had become a public critic.

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For many sons and daughters who came to maturity in the ’50s, the conservative climate of the Federal Republic of Germany could be stifling. For Habermas, intellectual guidance came from abroad when Theodor Adorno and Max Horkheimer—the two leading members of the Marxist-oriented Institute for Social Research, who had spent the war years in the United States—returned from exile. Habermas corresponded with Adorno for the first time in December 1955 and became his personal assistant and an official member of the newly refounded institute in early 1956. The philosophical intensity of the Frankfurt scene inspired him. The stated aims of the institute spoke to his own emergent belief that philosophy could best succeed in a multidisciplinary alliance with sociology. “Critical theory” meant opposing the mythologies of fascism and reawakening the repressed energies of enlightenment. In an early radio program, Habermas extolled “the Jewish heritage drawn from the German spirit” as a gift that was now “indispensable for our own life and survival.” But he found the culture at the institute somewhat exotic. “I felt like a figure from a novel by Balzac,” Habermas recalled, “the awkward and uneducated boy from the country whose eyes are opened by the city.” Adorno embodied a style of German-Jewish erudition that the Nazis had done their best to destroy. “Time,” Habermas noted, “had two dimensions.” The institute was thoroughly modern but also a remnant of the past.

Habermas is rightly seen as an incarnation of the Frankfurt School in its “second generation.” But in intellectual temperament, he is quite distinct from his teachers. From the start, his associations with Adorno were very warm, but he had difficulty with Horkheimer, who regarded him with suspicion and feared that the young man’s public criticism of the West German rearmament program would bring the institute into disrepute. Many years later, following Horkheimer’s death in 1973, Habermas was able to read the full text of a private letter to Adorno in which Horkheimer denounced their student as a revolutionary who would help “the gentlemen in the East.” Horkheimer demanded that Habermas resign. Adorno refused, but relations between Horkheimer and Habermas remained strained. In 1959, Habermas left the institute—a risky decision for a young scholar who was now married with two children. His wife, Ute, was surprised. But with the support of the philosopher Hans-Georg Gadamer, Habermas received a fellowship to complete his habilitation on the idea of the “public sphere,” and in 1961 he secured an assistant professorship in philosophy at the University of Heidelberg.

The Transformation of the Public Sphere, published a year later, contained many of the themes that Habermas would develop later in his career. It is often mischaracterized as a purely historical study because it takes note of bourgeois institutions like newspapers and coffeehouses. But the truth is that the book combines historical sociology with philosophy; it traces both the genesis and betrayal of a certain ideal of critical publicity that the European bourgeoisie held out as a promise and yet could never fully realize, thanks to the power and property constraints of bourgeois society. If there’s a single polestar for this argument, it is Kant’s ideal of the Enlightenment as an “age of criticism.” But its Marxist tonalities are no less profound: Critics who find Habermas too credulous in his devotion to public reason miss the fact that his book ends with a grim comment on the “refeudalization” of the public sphere. With the rise of mass-media forms and the distorting influence of money on communication, the ideal of rational criticism that once helped the bourgeoisie wrest itself free from the ancien régime now threatens to dissolve. Publicity, Habermas warns, has begun to resemble once again what it was in the feudal age: a mere performance.

This argument pays homage to the earlier 1944 study by Adorno and Horkheimer, Dialectic of Enlightenment, which explores the simultaneously emancipatory and oppressive impact of reason in world history. But whereas the earlier book prosecutes its case with literary allusion and bracing abstraction, Habermas takes care to sustain a bond with social reality. It is this sober and more realistic strain in his thinking that most characterizes his mature work as both a social theorist and a public intellectual.

Throughout the 1960s, Habermas’s reputation grew. In 1961, he participated in the famous “positivism dispute” and openly criticized Karl Popper for cultivating a form of “bisected rationalism” that dispensed with moral-political concerns and delimited reason’s scope to natural-scientific and technical affairs. In 1964, upon Horkheimer’s retirement, Habermas accepted an offer to assume his teacher’s chair in philosophy at Goethe University in Frankfurt. Memories of these times give the impression of a brilliant professor who possessed a healthy dose of self-deprecating humor. During a lecture in the university’s largest auditorium, a student interrupted to ask if Habermas “could express himself a little less complicatedly, for it was so difficult to understand him. One half of the audience applauded. He promised to do his best in order to be intelligible, Habermas replied, whereupon the other half of the audience started booing. To those who were now booing, Habermas continued, he could promise that his good intentions were bound to fail.”

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Formidable in his scholarship, Habermas had become, by the late 1960s, a bold and sometimes controversial voice in German politics. He signed a declaration to West German Chancellor Ludwig Erhard that demanded an end to the bombing of Vietnam, and he criticized the national leaders of the Social Democratic Party (including Willy Brandt) for their readiness to compromise with conservatives. In the summer of 1967, the police shooting of the 27-year-old Benno Ohnesorg led to student demonstrations across the Federal Republic. Following the young man’s funeral, Habermas spoke at a conference in defense of the student movement: “It was and still is the task of the student opposition to compensate for the lack of theoretical perspective, the lack of critical awareness of cover-ups and the branding of others as heretics, the lack of radicalness in the interpretation and implementation of our social and democratic constitution.” His support for the students, however, was not absolute. When Rudi Dutschke, a leader of the student radicals, alluded to “alternative forms of action” and “the possible use of violence,” Habermas warned that a “voluntaristic ideology” could easily slip from utopianism into what he called “left-wing fascism.” The phrase carried a considerable sting, especially for young radicals who had cultivated a self-image as opponents of the fascist past. A decade later, Habermas himself would say that he had “reacted a tad too much as a bourgeois intellectual.” But in retrospect, his warning seems prescient: By the late 1970s, a group of leftist militants had split off into the so-called Red Army Faction in association with Andreas Baader and Ulrike Meinhof and descended into acts of arson, kidnapping, and murder. The moral derangement of these years was only enhanced when German conservatives seized the opportunity to level against Habermas the absurd charge that he had furnished a “left-wing theory” for terrorism.

The “German Autumn” of 1977 ushered in a new decade of conservative ascendancy under the chancellorship of Helmut Kohl. In 1985, when Ronald Reagan visited West Germany on the occasion of the 40th anniversary of the Allies’ victory in World War II, he first visited Bergen-Belsen (the site of a former concentration camp) and then paid a visit to a military cemetery near the town of Bitburg, where Kohl had organized a ceremony to commemorate the dead. Among the graves of German soldiers were those for members of the Waffen SS. Stunned by the offense, Habermas published an essay in Die Zeit in which he accused Kohl of a “disposal of the past.” The following year brought the opening phases of the Historikerstreit, or “historians’ dispute,” when Habermas identified conservative and nationalist trends in contemporary historiography that sought to relativize the crimes of the Third Reich. After the collapse of the Communist bloc, as West Germany pressed toward reunification with its eastern neighbor, Habermas again feared a resurgent nationalism that would subjugate democracy to economic might. “German interests,” he lamented, were being “weighed and enforced in German Marks.”

In an age of mass media, those who style themselves as public intellectuals often seem more interested in performance than in genuine dialogue. For Habermas, however, true criticism is possible only in the volatile exchange of arguments between subjects. This commitment to the ideal of rational communication, undistorted by asymmetries of power, received its most elaborate treatment in 1981, when he published the monumental two-volume Theory of Communicative Action. The fruit of a full decade of research and over 1,000 pages in length, the book signaled what some have called a “linguistic turn” in critical theory. But the term can be misleading. Inspired by extensive reading in American pragmatism (especially the semiotics of Charles Sanders Peirce), Habermas’s inquiry into what he and his colleague Karl-Otto Apel described as the “formal pragmatics” of language did not entail a turning of attention away from society. The rationality that Habermas sets out to explore is a kind of immanent utopia, the delicate filigree of rational communication that we fashion with others and rely on whenever we aim toward mutual understanding. It is a social creation—intersubjective, not merely instrumental—and its telos is the unforced ideal of agreement. Skeptics may respond that Habermas expects too much from human reason, but his crucial insight is difficult to evade without falling into self-contradiction. The moment one opts for genuine communication, one has already taken on board the expectation of intelligibility. Every act of communication, however, is a potential risk, leaving itself exposed to criticism. But this is what gives language its democratic edge. For Heidegger, language was the “house of Being,” a conservative idea that ceded to language an anonymous power beyond human amendment or appeal. For Habermas, however, language is a fragile and cooperative project that comes alive only in the space between subjects. Open-ended and potentially universal in its reach, it is the house of mundane reason.

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Critics sometimes mistake Habermas for a cool-minded logician when, in fact, his rationalism entails a richly textured theory of modern society. Borrowing from both Weber and Marx along with insights from anthropology, Habermas proposes that we think of society as a dynamic union between the “lifeworld” (our ongoing project of communication through which we reproduce cultural meaning) and the “system” (the infrastructure of economy and administration that gives the social order its stability). Ideally, the system serves the lifeworld: Through the process of rational deliberation, we offer proposals and counterproposals for how we might restructure society, and we modify the system as we go along with the expectation that all policies remain open to criticism and revision. The lifeworld is the place of inherited meaning, but also the site where old meanings are scrutinized and dismantled. Unlike poststructuralists such as Foucault, who see language as a network of power that conditions all possibility, Habermas looks to language as a solvent against the calcified structures of tradition. Discourse “is not an institution,” he explains; “it is a counter-institution par excellence.”

But Habermas is a realist about the social utility of the system. Although he still draws a great deal from the Marxist critique of capitalism, he is at heart a reformist social democrat who has made his peace with the endurance of private property and the bureaucratic structures of the modern welfare state. The problem, in his view, is that all too often the fragile lines of communication are distorted or even broken by economic interests—and when this happens, the system can take on the opacity and independence of merely technical imperatives that resist our control. The imperatives of the system can then “colonize” the lifeworld, resulting in a condition that Marx called “reification”: Society confronts us like a frozen and uncanny thing, no longer a reflection of our collective aims.

While working on the theory of communicative action, Habermas served as director of the Max Planck Institute in Starnberg, a wealthy suburb some 30 kilometers from Munich, where he and his family also made their home. In 1981, he resigned from the institute and took up the position as professor of philosophy in Frankfurt. On his office wall, he hung a photograph of Adorno, who died in 1969 and whom Habermas has called “the only genius I have met in my life.”

For the last three and a half decades, Habermas’s interests have only expanded. His 1985 work, The Philosophical Discourse of Modernity, brought him into confrontation with poststructuralist readers, especially in North America, who did not welcome his criticism of luminaries like Derrida and Foucault. Two years before, however, Habermas had met privately with Foucault on several occasions in Paris, during which time at least some of his philosophical misgivings about his French colleague had dissipated. Upon Foucault’s death in June 1984, Habermas published a tribute to the philosopher as a partner in thinking through the legacies of the Enlightenment, the theme for a conference they’d been planning together.

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In the years since Germany’s reunification, Habermas has increasingly turned his attention to questions of political theory, law, and religion. In Between Facts and Norms (1992), he sought to fill in the institutional basis of his “discourse ethics” by rethinking the relationship between legality and democracy. Although he officially retired in 1994, even today he sustains remarkable energy both as a scholar and a critic in the public sphere. The first years of the new millennium revivified his concern for the future of the European Union and drew him into the international controversy over the United States’ military attack on Iraq. In February 2003, huge crowds throughout Europe’s major cities demonstrated against the Bush administration’s planned invasion. That May, just after Bush engineered his public-relations stunt of landing on an aircraft carrier decked out with a “Mission Accomplished” banner, Habermas coauthored a statement with Derrida, published simultaneously in German in the Frankfurter Allgemeine Zeitung and in French in Libération, in which they referred to the February demonstrations as a sign of “the birth of a European public.” Europe, they wrote, “must throw its weight onto the scales at the international level and within the UN in order to counterbalance the hegemonic unilateralism of the United States. At global economic summits and in the institutions of the WTO [World Trade Organization], the World Bank, and the International Monetary Fund, it should bring its influence to bear in shaping the design of a future global domestic politics.”

The recent crises of the EU, both financial and political, haven’t shaken Habermas’s belief in strengthening the institutions of democratic solidarity that can permit the emergence of a truly European identity. The proper response to globalization, in his view, is not nationalistic withdrawal (as in the recent Brexit vote), but enhanced regulation and a support for horizontal procedures of decision-making against the “executive federalism” imposed from above by policy leaders in both Germany and France. Even today, Habermas hasn’t ceased to defend Kant’s ideal of a genuinely cosmopolitan public sphere in which the bonds of reason transcend the old boundaries of national belonging.

In the penultimate chapter of the new biography, Müller-Doohm makes explicit the comparison to Kant when he offers a helpful anatomy of the four major themes of Habermas’s work according to the “four questions” that Kant, in his Lectures on Logic, sees as defining “philosophy in this cosmopolitan sense”: “What can I know?,” “What should I do?,” “What can I hope for?,” and “What is a human being?” It is no exaggeration to say that Habermas has succeeded in demonstrating that these four questions are really one: We can only know the essence of our humanity if we sustain our commitment to the moral and political ideals that will permit humanity as a whole to realize its highest aspirations. Unlike Kant, Habermas is a thinker of late modernity; he no longer subscribes to the lofty belief in philosophy as the “queen of the sciences.” Instead, as a critical theorist in the tradition of his teachers, he embraces a conception of “post-metaphysical thinking” that sustains an alliance with the rest of the human sciences and remains responsive to its own social-historical context. Although genuinely metaphysical knowledge is no longer the rightful province of philosophical speculation, Habermas still cleaves in his own way to what Adorno once called “metaphysics at the moment of its fall.” In our capacity for rational communication and in our appeal to a morality that leaves no one behind, there lies (in Habermas’s phrase) “a moment of unconditionality.” While it lacks the prestige of a metaphysical absolute, it still bears a trace of the older idealism. Habermas calls it “an absolute that has become fluid as a critical procedure.” Mundane reason, in other words, isn’t wholly mundane: In its modest commitment to rational argumentation, it keeps alive the universalizing impulse of the monotheistic religions when it strives to break free of its own conditions and “points beyond all particular forms of life.”

We live in an age of such deflated expectations that too many of us are now ready to equate intellect with cynicism, as if we could hope for little more from public intellectuals than irony and polemic. We owe to Habermas an enduring model of fidelity to a higher ideal. Among the many honors that he has received over the course of his career, the Kyoto Prize in 2004 served as an occasion for Habermas to affirm his faith in the standards of communicative reason. “It is hardly surprising that we fail to live up to these standards,” he observed, “but that in no way devalues the standards themselves. For if there is one thing that intellectuals—a species that has so often attacked its own kind and pronounced its demise—cannot allow themselves…it is to become cynical.”