Obscurantism is the practice of deliberately preventing the facts of some subject matter from becoming known. There are two historical and intellectual denotations of Obscurantism: (1) the deliberate restriction of knowledge—opposition to disseminating knowledge; and, (2) deliberate obscurity—an abstruse style (as in literature and art) characterized by deliberate vagueness. Therefore, an obscurantist is someone who actively opposes enlightenment and the consequent social reform, a type of anti-intellectual.

The term obscurantism derives from the title of the 16th-century satire Epistolæ Obscurorum Virorum (1515–19, Letters of Obscure Men), that was based upon the intellectual dispute between the German humanist Johann Reuchlin and the monk Johannes Pfefferkorn of the Dominican Order.

In the 18th century, Enlightenment philosophers applied the term obscurantist to any enemy of intellectual enlightenment and the liberal diffusion of knowledge. In the 19th century, in distinguishing the varieties of obscurantism found in metaphysics and theology from the "more subtle" obscurantism of the critical philosophy of Immanuel Kant, and of modern philosophical skepticism, Friedrich Nietzsche said: "The essential element in the black art of obscurantism is not that it wants to darken individual understanding, but that it wants to blacken our picture of the world, and darken our idea of existence.

In restricting knowledge to an élite ruling class of "the few", obscurantism is fundamentally anti-democratic, because its component anti-intellectualism and elitism exclude the people as intellectually unworthy of knowing the facts and truth about the government of their City-State. In 18th century monarchic France, the Marquis de Condorcet, as a political scientist, documented the aristocracy's obscurantism about the social problems that provoked the French Revolution (1789–99) that deposed them and their King, Louis XVI of France.

The obscurantist can be a scientist, a philosopher, a faithful person, an atheist, a student, or an agnostic, but, as a member of society, he or she believes that religion serves the social control of the populace. To that effect, the obscurantist limits the publication, extension, and dissemination of knowledge, of evidence countering the common-belief status quo with which the nation are ruled—the local variety of “the necessary” Noble Lie, which concept was introduced to political discourse by the Classical Greek philosopher Plato, in 380 BC. Hence, the stable–status quo restriction of knowledge definition of obscurantism applied by pro-science reformers within religious movements, and by skeptics, such as H.L. Mencken, in critiquing religion.

The second sense of obscurantism denotes making knowledge abstruse, that is, difficult to grasp. In the 19th and 20th centuries obscurantism became a polemical term for accusing an author of deliberately writing obscurely, in order to hide his or her intellectual vacuousness. Philosophers who are neither empiricists nor positivists often are considered obscurantists when describing the abstract concepts of their disciplines. For philosophic reasons, such authors might modify or reject verifiability, falsifiability, and logical non-contradiction. From that perspective, obscure (clouded, vague, abstruse) writing does not necessarily indicate that the writer has a poor grasp of the subject, because unintelligible writing sometimes is purposeful and philosophically considered.

Aristotle: In contemporary discussions of virtue ethics, Aristotle's Nicomachean Ethics (The Ethics) stands accused of ethical obscurantism, because of the technical, philosophic language and writing style, and their purpose being the education of a cultured governing elite.

Kant: Immanuel Kant employed technical terms that were not commonly understood, by the layman. Arthur Schopenhauer contended that post-Kantian philosophers such as Johann Gottlieb Fichte, Friedrich Wilhelm Joseph Schelling, and Georg Wilhelm Friedrich Hegel deliberately imitated the abstruse style of writing practised by Kant. "Because of his style which was obscure, Kant was properly understood by exceedingly few. And it is as if all the philosophical writers, who since Kant had had some success, had devoted themselves to writing still more unintelligibly than Kant. This was bound to succeed!"

Hegel: G. W. F. Hegel's philosophy, and the philosophies of those he influenced, especially Karl Marx, have been accused of obscurantism. Analytic and positivistic philosophers, such as A. J. Ayer, Bertrand Russell, and the critical-rationalist Karl Popper, accused Hegel and Hegelianism of being obscure. About Hegel's philosophy, Schopenhauer wrote that it is: "... a colossal piece of mystification, which will yet provide posterity with an inexhaustible theme for laughter at our times, that it is a pseudo-philosophy paralyzing all mental powers, stifling all real thinking, and, by the most outrageous misuse of language, putting in its place the hollowest, most senseless, thoughtless, and, as is confirmed by its success, most stupefying verbiage. ..." Nevertheless, biographer Terry Pinkard notes "Hegel has refused to go away, even in analytic philosophy, itself." Hegel was aware of his obscurantism, and perceived it as part of philosophical thinking—to accept and transcend the limitations of quotidian thought and its concepts. In the essay "Who Thinks Abstractly?", he said that it is not the philosopher who thinks abstractly, but the layman, who uses concepts as givens that are immutable, without context. It is the philosopher who thinks concretely, because he transcends the limits of quotidian concepts, in order to understand their broader context. This makes philosophical thought and language appear obscure, esoteric, and mysterious to the layman.

Marx: Karl Marx in 1861 In his early works, Karl Marx criticized German and French philosophy, especially German Idealism, for its traditions of German irrationalism and ideologically motivated obscurantism. Later thinkers whom he influenced, such as the philosopher György Lukács and the sociologist Jürgen Habermas, followed with similar arguments of their own. However, philosophers such as Karl Popper and Friedrich Hayek in turn criticized Marx and Marxist philosophy as obscurantist (however, see above for Hayek's particular interpretation of the term).

Heidegger: Martin Heidegger, and those influenced by him, such as Jacques Derrida and Emmanuel Levinas, have been labeled obscurantists by critics from analytic philosophy and the Frankfurt School of critical theory. Of Heidegger, Bertrand Russell wrote, "his philosophy is extremely obscure. One cannot help suspecting that language is here running riot. An interesting point in his speculations is the insistence that nothingness is something positive. As with much else in Existentialism, this is a psychological observation made to pass for logic." That is Russell's complete entry on Heidegger, and it expresses the sentiments of many 20th-century analytic philosophers concerning Heidegger.

Derrida: In their obituaries, "Jacques Derrida, Abstruse Theorist, Dies at 74" (10 October 2004) and "Obituary of Jacques Derrida, French intellectual" (21 October 2004), The New York Times newspaper and The Economist magazine, described Derrida as a deliberately obscure philosopher. In Contingency, Irony, and Solidarity (1989), Richard Rorty proposed that in The Post Card: From Socrates to Freud and Beyond (1978), Jacques Derrida purposefully used undefinable words (e.g. Différance), and used defined words in contexts so diverse that they render the words unintelligible, hence, the reader is unable to establish a context for his literary self. In that way, the philosopher Derrida escapes metaphysical accounts of his work. Since the work ostensibly contains no metaphysics, Derrida has, consequently, escaped metaphysics. Derrida's philosophic work is especially controversial among American and British academics, as when the University of Cambridge awarded him an honorary doctorate, despite opposition from among the Cambridge philosophy faculty and analytical philosophers worldwide. In opposing the decision, philosophers including Barry Smith, W. V. O. Quine, David Armstrong, Ruth Barcan Marcus, René Thom, and twelve others, published a letter of protestation in The Times of London, arguing that "his works employ a written style that defies comprehension . . . [thus] Academic status based on what seems to us to be little more than semi-intelligible attacks upon the values of reason, truth, and scholarship is not, we submit, sufficient grounds for the awarding of an honorary degree in a distinguished university."

In the New York Review of Books article "An Exchange on Deconstruction" (February 1984), John Searle comments on Deconstruction: ". . . anyone who reads deconstructive texts with an open mind is likely to be struck by the same phenomena that initially surprised me: the low level of philosophical argumentation, the deliberate obscurantism of the prose, the wildly exaggerated claims, and the constant striving to give the appearance of profundity, by making claims that seem paradoxical, but under analysis often turn out to be silly or trivial."

Lacan: Jacques Lacan was an intellectual who defended obscurantism to a degree. To his students' complaint about the deliberate obscurity of his lectures, he replied: "The less you understand, the better you listen." In the 1973 seminar Encore, he said that his Écrits (Writings) were not to be understood, but would effect a meaning in the reader, like that induced by mystical texts. The obscurity is not in his writing style, but in the repeated allusions to Hegel, derived from Alexandre Kojève's lectures on Hegel, and similar theoretic divergences.

Nietsche: Friedrich Nietsche said: "The essential element in the black art of obscurantism is not that it wants to darken individual understanding, but that it wants to blacken our picture of the world, and darken our idea of existence" (1878) Human, All Too Human Vol. II, Part 1, 27. Cambridge University Press; 2 edition (November 13, 1996). ISBN 978-0-521-56704-6).