Rationalism is a philosophical doctrine that posits reason as the chief source and test of knowledge, emphasizing the role of innate ideas and deductive reasoning over sensory experience. Its origins can be traced back to ancient Greek philosophers such as Pythagoras and Plato, who believed that reality has a rational structure accessible through intellectual insight rather than empirical observation.

The modern form of rationalism emerged during the 17th century with thinkers such as René Descartes, Baruch Spinoza, and Gottfried Wilhelm Leibniz. Descartes, often considered the father of modern philosophy, introduced methodological skepticism and the famous dictum “Cogito, ergo sum” (“I think, therefore I am”), asserting the certainty of self-awareness as the foundational truth. Spinoza expanded on rationalist ideas by proposing a monistic universe where God and nature are one, comprehensible through reason. Leibniz contributed the concept of pre-established harmony and the idea that the universe consists of simple substances known as monads, which operate according to logical principles.

During the Enlightenment, rationalism significantly influenced scientific inquiry and political thought, promoting ideals of progress, secularism, and individual rights. It encouraged a departure from traditional authorities and religious dogma, fostering advancements in mathematics, physics, and philosophy. However, rationalism also faced critiques from empiricists such as John Locke and David Hume, who argued that knowledge derives from sensory experience and that the mind starts as a blank slate.

In the 19th and 20th centuries, rationalist principles were further developed and challenged. Immanuel Kant attempted to reconcile rationalism and empiricism by proposing that while all knowledge begins with experience, not all knowledge arises from experience. Later philosophical movements, such as logical positivism and analytic philosophy, drew upon rationalist ideas to explore language, logic, and scientific methodology. Today, rationalism continues to influence debates in epistemology, ethics, and the philosophy of science, remaining a fundamental perspective in Western thought.