René Descartes, after Frans Hals, 2nd half of the 17th century.

Descartes laid the foundation for 17th-century continental rationalism, later advocated by Baruch Spinoza and Gottfried Leibniz, and opposed by the empiricist school of thought consisting of Hobbes, Locke, Berkeley, and Hume. Leibniz, Spinoza, and Descartes were all well-versed in mathematics, as well as philosophy, and Descartes and Leibniz contributed greatly to science as well.

Rationalism v. Empiricism

Since the Enlightenment, rationalism is usually associated with the introduction of mathematical methods into philosophy, as seen in the works of Descartes, Leibniz, and Spinoza. This is commonly called continental rationalism, because it was predominant in the continental schools of Europe, whereas in Britain, empiricism, or a theory that knowledge comes only or primarily from a sensory experience, dominated. Although rationalism and empiricism are traditionally seen as opposing each other, the distinction between rationalists and empiricists was drawn at a later period, and would not have been recognized by philosophers involved in Enlightenment debates. Furthermore, the distinction between the two philosophies is not as clear-cut as is sometimes suggested. For example, Descartes and John Locke, one of the most important Enlightenment thinkers, have similar views about the nature of human ideas.

Proponents of some varieties of rationalism argue that, starting with foundational basic principles, like the axioms of geometry, one could deductively derive the rest of all possible knowledge. The philosophers who held this view most clearly were Baruch Spinoza and Gottfried Leibniz, whose attempts to grapple with the epistemological and metaphysical problems raised by Descartes led to a development of the fundamental approach of rationalism. Both Spinoza and Leibniz asserted that, in principle, all knowledge, including scientific knowledge, could be gained through the use of reason alone, though they both observed that this was not possible in practice for human beings, except in specific areas, such as mathematics. On the other hand, Leibniz admitted in his book, Monadology, that “we are all mere Empirics in three fourths of our actions.”

Immanuel Kant

Descartes, Spinoza, and Leibniz are usually credited for laying the groundwork for the 18th-century Enlightenment. During the mature Enlightenment period, Immanuel Kant attempted to explain the relationship between reason and human experience, and to move beyond the failures of traditional philosophy and metaphysics. He wanted to put an end to an era of futile and speculative theories of human experience, and regarded himself as ending and showing the way beyond the impasse between rationalists and empiricists. He is widely held to have synthesized these two early modern traditions in his thought.

Kant named his brand of epistemology (theory of knowledge) “transcendental idealism,” and he first laid out these views in his famous work, The Critique of Pure Reason. In it, he argued that there were fundamental problems with both rationalist and empiricist dogma. To the rationalists he argued, broadly, that pure reason is flawed when it goes beyond its limits and claims to know those things that are necessarily beyond the realm of all possible experience (e.g., the existence of God, free will, or the immortality of the human soul). To the empiricist, he argued that while it is correct that experience is fundamentally necessary for human knowledge, reason is necessary for processing that experience into coherent thought. He therefore concluded that both reason and experience are necessary for human knowledge. In the same way, Kant also argued that it was wrong to regard thought as mere analysis. In his views, a priori concepts do exist, but if they are to lead to the amplification of knowledge, they must be brought into relation with empirical data.

Immanuel Kant, author unknown Immanuel Kant (1724-1804) rejected the dogmas of both rationalism and empiricism, and tried to reconcile rationalism and religious belief, and individual freedom and political authority, as well as map out a view of the public sphere through private and public reason. His work continued to shape German thought, and indeed all of European philosophy, well into the 20th century. 

Politics

Since the Enlightenment, rationalism in politics historically emphasized a “politics of reason” centered upon rational choice, utilitarianism, and secularism (later, relationship between rationalism and religion was ameliorated by the adoption of pluralistic rationalist methods practicable regardless of religious or irreligious ideology). Some philosophers today, most notably John Cottingham, note that rationalism, a methodology, became socially conflated with atheism, a worldview. Cottingham writes,

In the past, particularly in the 17th and 18th centuries, the term ‘rationalist’ was often used to refer to free thinkers of an anti-clerical and anti-religious outlook, and for a time the word acquired a distinctly pejorative force (…). The use of the label ‘rationalist’ to characterize a world outlook which has no place for the supernatural is becoming less popular today; terms like ‘humanist’ or ‘materialist’ seem largely to have taken its place. But the old usage still survives.

Attributions