a 19th- and early 20th-century philosophical trend that claimed to enrich man's apprehension of life by expanding it beyond the rational to its fuller dimensions. Rooted either in metaphysics or in an awareness of the uniqueness of human experience, irrationalism stressed the dimensions of instinct, feeling, and will as over and against reason. The term is used chiefly by continental European philosophers, who regard irrationalism as one of several strong currents flowing into the 20th century.

There were irrationalists before the 19th century. In ancient Greek culture—which is usually assessed as rationalistic—a Dionysian (i.e., instinctive) strain can be discerned in the works of the poet Pindar, in the dramatists, and even in such philosophers as Pythagoras and Empedocles and in Plato. In early modern philosophy—even during the ascendancy of Cartesian rationalism—Blaise Pascal turned from reason to an Augustinian faith, convinced that “the heart has its reasons” unknown to reason as such.

The main tide of irrationalism, like that of literary romanticism—itself a form of irrationalism—followed the Age of Reason and was a reaction to it. Irrationalism found much in the life of the spirit and in human history that could not be dealt with by the rational methods of science. Under the influence of Charles Darwin and later Sigmund Freud, irrationalism began to explore the biological and subconscious roots of experience. Pragmatism, existentialism, and vitalism (or “life philosophy”) all arose as expressions of this expanded view of human life and thought.

For Arthur Schopenhauer (Schopenhauer, Arthur), a typical 19th-century irrationalist, voluntarism expressed the essence of reality—a blind, purposeless will permeating all existence. If mind, then, is an emergent from mute biological process, it is natural to conclude, as the pragmatists did, that it evolved as an instrument for practical adjustment—not as an organ for the rational plumbing of metaphysics. Charles Sanders Peirce (Peirce, Charles Sanders) and William James (James, William) thus argued that ideas are to be assessed not in terms of logic but in terms of their practical results when put to the test of action.

Irrationalism is also expressed in the historicism and relativism of Wilhelm Dilthey (Dilthey, Wilhelm), who saw all knowledge as conditioned by one's private historical perspective and who thus urged the importance of the Geisteswissenschaften (the humanities). Johann Georg Hamann (Hamann, Johann Georg), spurning speculation, sought truth in feeling, faith, and experience, making personal convictions its ultimate criterion. Friedrich Heinrich Jacobi (Jacobi, Friedrich Heinrich) exalted the certitude and clarity of faith to the detriment of intellectual knowledge and sensation.

Friedrich Schelling and Henri Bergson (Bergson, Henri), who were preoccupied with the uniqueness of human experience, turned to intuitionism, which “sees things invisible to science.” Reason itself was not repudiated; it had simply lost its commanding role inasmuch as personal insights are impervious to testing. In its aspect as a vitalism, Bergson's philosophy—as well as that of Friedrich Nietzsche (Nietzsche, Friedrich)—was irrationalistic in holding that instinctive, or Dionysian, drive lies at the heart of existence. Nietzsche viewed moral codes as myths, lies, and frauds created to mask forces operating beneath the surface to influence thought and behaviour. For him, God is dead and man is free to formulate new values. Ludwig Klages (Klages, Ludwig) extended life philosophy in Germany by urging that the irrational springs of human life are “natural” and should be followed in a deliberate effort to root out the adventitious reason; and Oswald Spengler (Spengler, Oswald) extended it to history, which he viewed intuitively as an irrational process of organic growth and decay.

In Existentialism, Søren Kierkegaard, Jean-Paul Sartre, and Albert Camus all despaired of making sense out of an incoherent world; and each chose his own alternative to reason—the leap of faith, radical freedom, and heroic revolt, respectively.

In general, irrationalism implies either (in ontology) that the world is devoid of rational structure, meaning, and purpose; or (in epistemology) that reason is inherently defective and incapable of knowing the universe without distortion; or (in ethics) that recourse to objective standards is futile; or (in anthropology) that in human nature itself the dominant dimensions are irrational.