Along with Ludwig Wittgenstein and Martin Heidegger, Jacques Derrida, who died last week in Paris at the age of 74, will be remembered as one of the three most important philosophers of the 20th century. No thinker in the last 100 years had a greater impact than he did on people in more fields and different disciplines. Philosophers, theologians, literary and art critics, psychologists, historians, writers, artists, legal scholars and even architects have found in his writings resources for insights that have led to an extraordinary revival of the arts and humanities during the past four decades. And no thinker has been more deeply misunderstood. To people addicted to sound bites and overnight polls, Mr. Derrida's works seem hopelessly obscure. It is undeniable that they cannot be easily summarized or reduced to one-liners. The obscurity of his writing, however, does not conceal a code that can be cracked, but reflects the density and complexity characteristic of all great works of philosophy, literature and art. Like good French wine, his works age well. The more one lingers with them, the more they reveal about our world and ourselves. What makes Mr. Derrida's work so significant is the way he brought insights of major philosophers, writers, artists and theologians to bear on problems of urgent contemporary interest. Most of his infamously demanding texts consist of careful interpretations of canonical writers in the Western philosophical, literary and artistic traditions—from Plato to Joyce. By reading familiar works against the grain, he disclosed concealed meanings that created new possibilities for imaginative expression. Mr. Derrida's name is most closely associated with the often cited but rarely understood term "deconstruction." Initially formulated to define a strategy for interpreting sophisticated written and visual works, deconstruction has entered everyday language. When responsibly understood, the implications of deconstruction are quite different from the misleading clichïs often used to describe a process of dismantling or taking things apart. The guiding insight of deconstruction is that every structure—be it literary, psychological, social, economic, political or religious—that organizes our experience is constituted and maintained through acts of exclusion. In the process of creating something, something else inevitably gets left out. These exclusive structures can become repressive—and that repression comes with consequences. In a manner reminiscent of Freud, Mr. Derrida insists that what is repressed does not disappear but always returns to unsettle every construction, no matter how secure it seems. As an Algerian Jew writing in France during the postwar years in the wake of totalitarianism on the right (fascism) as well as the left (Stalinism), Mr. Derrida understood all too well the danger of beliefs and ideologies that divide the world into diametrical opposites: right or left, red or blue, good or evil, for us or against us. He showed how these repressive structures, which grew directly out of the Western intellectual and cultural tradition, threatened to return with devastating consequences. By struggling to find ways to overcome patterns that exclude the differences that make life worth living, he developed a vision that is consistently ethical. And yet, supporters on the left and critics on the right have misunderstood this vision. Many of Mr. Derrida's most influential followers appropriated his analyses of marginal writers, works and cultures as well as his emphasis on the importance of preserving differences and respecting others to forge an identity politics that divides the world between the very oppositions that it was Mr. Derrida's mission to undo: black and white, men and women, gay and straight. Betraying Mr. Derrida's insights by creating a culture of political correctness, his self-styled supporters fueled the culture wars that have been raging for more than two decades and continue to frame political debate. To his critics, Mr. Derrida appeared to be a pernicious nihilist who threatened the very foundation of Western society and culture. By insisting that truth and absolute value cannot be known with certainty, his detractors argue, he undercut the very possibility of moral judgment. To follow Mr. Derrida, they maintain, is to start down the slippery slope of skepticism and relativism that inevitably leaves us powerless to act responsibly. This is an important criticism that requires a careful response. Like Kant, Kierkegaard and Nietzsche, Mr. Derrida does argue that transparent truth and absolute values elude our grasp. This does not mean, however, that we must forsake the cognitive categories and moral principles without which we cannot live: equality and justice, generosity and friendship. Rather, it is necessary to recognize the unavoidable limitations and inherent contradictions in the ideas and norms that guide our actions, and do so in a way that keeps them open to constant questioning and continual revision. There can be no ethical action without critical reflection. During the last decade of his life, Mr. Derrida became preoccupied with religion and it is in this area that his contribution might well be most significant for our time. He understood that religion is impossible without uncertainty. Whether conceived of as Yahweh, as the father of Jesus Christ, or as Allah, God can never be fully known or adequately represented by imperfect human beings. And yet, we live in an age when major conflicts are shaped by people who claim to know, for certain, that God is on their side. Mr. Derrida reminded us that religion does not always give clear meaning, purpose and certainty by providing secure foundations. To the contrary, the great religious traditions are profoundly disturbing because they all call certainty and security into question. Belief not tempered by doubt poses a mortal danger. As the process of globalization draws us ever closer in networks of communication and exchange, there is an understandable longing for simplicity, clarity and certainty. This desire is responsible, in large measure, for the rise of cultural conservatism and religious fundamentalism—in this country and around the world. True believers of every stripe—Muslim, Jewish and Christian—cling to beliefs that, Mr. Derrida warns, threaten to tear apart our world. Fortunately, he also taught us that the alternative to blind belief is not simply unbelief but a different kind of belief—one that embraces uncertainty and enables us to respect others whom we do not understand. In a complex world, wisdom is knowing what we don't know so that we can keep the future open. In the two decades I knew Mr. Derrida, we had many meetings and exchanges. In conversation, he listened carefully and responded helpfully to questions whether posed by undergraduates or colleagues. As a teacher, he gave freely of his time to several generations of students. But small things are the measure of the man. In 1986, my family and I were in Paris and Mr. Derrida invited us to dinner at his house in the suburbs 20 miles away. He insisted on picking us up at our hotel, and when we arrived at his home he presented our children with carnival masks. At 2 a.m., he drove us back to the city. In later years, when my son and daughter were writing college papers on his work, he sent them letters and postcards of encouragement as well as signed copies of several of his books. Jacques Derrida wrote eloquently about the gift of friendship but in these quiet gestures—gestures that served to forge connections among individuals across their differences—we see deconstruction in action.