Irrationalism is a philosophical movement that emerged in the early 19th century, emphasizing the non-rational dimension of human life. As they reject logic, irrationalists argue that instinct and feelings are superior to reason in the research of knowledge. The term has often been used as a pejorative designation of criticisms against rationalism as a whole.

The philosophy of rationalism, understood as having first emerged in the writings of Francis Bacon and René Descartes, has received a variety of criticisms since its inception. These may entail a view that certain things are beyond rational understanding, that total rationality is insufficient or even harmful to human life, or that people are not instinctively rational and progressive.

The term "irrationalism" does not refer to an independent philosophical movement, but is a moment and component of various philosophical movements and systems. Irrationalism in the true sense refers to worldviews that are particularly characterized by the moment of irrationality and that also put rational thinking aside in favor of alternative, higher cognitive functions, often in favor of a certain form of intuition. In this respect, the term has found a more specific use for certain philosophical positions.

The core tenent of irrationalism is to be opposed to philosophy of rationalism. Since the term irrationalism is often used as a derogatory accusation to criticize other positions as unreasonable, unscientific and thus wrong, it is controversial as a scientific category, especially in individual cases. Otherwise, however, the term is often used unspecifically and – like its counterpart, rationalism – in very different meanings.

Depending on the area in which theses on irrationalism are represented, one can distinguish between epistemological and ontological (sometimes metaphysical) positions of irrationalism. The rejection of rationality as the only source of meaningful knowledge has far-reaching effects on the assessment of scientific methodology. Therefore, irrationalist positions are often directed against exclusively rational scientific and social theories of development and progress. Traditions of philosophy are also specifically assessed according to these premises.

Irrationalism has its roots in ancient philosophy, with its foundational elements present in schools such as skepticism, sophism and neo-platonism.

The philosophy of rationalism, specifically classical rationalism, gained prominence during the Enlightenment, and was presented by the works and studies of René Descartes, Gottfried Leibniz, Baruch Spinoza and Immanuel Kant. Regardless, even during the Age of Enlightenment, these thinkers' notions were being already challenged.

Modern irrationalist positions are often traced back to their origins in German idealism and the Romantic movement. Johann Georg Hamann, Friedrich Heinrich Jacobi, and Friedrich Wilhelm Joseph von Schelling are noted pioneers of this philosophy. The common factor for the attribution is usually that of the intellectual views and aesthetic intuitions which Immanuel Kant was opposed to, given a special priority.