noun. Irrational thought, expression, or behavior; irrationality.

noun. Belief in feeling, instinct, or other nonrational forces rather than reason.

noun. The principles of a system of thought opposed to rationalism; irrationality; the quality of not being guided by reason.

noun. philosophical movement formed as a cultural reaction against positivism in the early 20th century

"For a bird's-eye view of the history of the idea of irrationalism in philosophy, two preliminary method - ological observations are in order."