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Rationalism is a philosophical movement which gathered momentum during the Age of Reason of the 17th Century. It is usually associated with the introduction of mathematical methods into philosophy during this period by the major rationalist figures, Descartes, Leibniz and Spinoza. The preponderance of French Rationalists in the 18th Century Age of Enlightenment, including Voltaire, Jean-Jacques Rousseau and Charles de Secondat (Baron de Montesquieu) (1689 - 1755), is often known as French Rationalism.

Rationalism is any view appealing to intellectual and deductive reason (as opposed to sensory experience or any religious teachings) as the source of knowledge or justification. Thus, it holds that some propositions are knowable by us by intuition alone, while others are knowable by being deduced through valid arguments from intuited propositions. It relies on the idea that reality has a rational structure in that all aspects of it can be grasped through mathematical and logical principles, and not simply through sensory experience.

Rationalists believe that, rather than being a "tabula rasa" to be imprinted with sense data, the mind is structured by, and responds to, mathematical methods of reasoning. Some of our knowledge or the concepts we employ are part of our innate rational nature: experiences may trigger a process by which we bring this knowledge to consciousness, but the experiences do not provide us with the knowledge itself, which has in some way been with us all along. See the section on the doctrine of Rationalism for more details.

Rationalism is usually contrasted with Empiricism (the view that the origin of all knowledge is sense experience and sensory perception), and it is often referred to as Continental Rationalism because it was predominant in the continental schools of Europe, whereas British Empiricism dominated in Britain. However, the distinction between the two is perhaps not as clear-cut as is sometimes suggested, and would probably not have even been recognized by the philosophers involved. Although Rationalists asserted that, in principle, all knowledge, including scientific knowledge, could be gained through the use of reason alone, they also observed that this was not possible in practice for human beings, except in specific areas such as mathematics.

It has some similarities in ideology and intent to the earlier Humanist movement in that it aims to provide a framework for philosophical discourse outside of religious or supernatural beliefs. But in other respects there is little to compare. While the roots of Rationalism may go back to the Eleatics and Pythagoreans of ancient Greece, or at least to Platonists and Neo-Platonists, the definitive formulation of the theory had to wait until the 17th Century philosophers of the Age of Reason.

René Descartes is one of the earliest and best known proponents of Rationalism, which is often known as Cartesianism (and followers of Descartes' formulation of Rationalism as Cartesians). He believed that knowledge of eternal truths (e.g. mathematics and the epistemological and metaphysical foundations of the sciences) could be attained by reason alone, without the need for any sensory experience. Other knowledge (e.g. the knowledge of physics), required experience of the world, aided by the scientific method - a moderate rationalist position. For instance, his famous dictum "Cogito ergo sum" ("I think, therefore I am") is a conclusion reached a priori and not through an inference from experience. Descartes held that some ideas (innate ideas) come from God; others ideas are derived from sensory experience; and still others are fictitious (or created by the imagination). Of these, the only ideas which are certainly valid, according to Descartes, are those which are innate.

Baruch Spinoza expanded upon Descartes' basic principles of Rationalism. His philosophy centred on several principles, most of which relied on his notion that God is the only absolute substance (similar to Descartes' conception of God), and that substance is composed of two attributes, thought and extension. He believed that all aspects of the natural world (including Man) were modes of the eternal substance of God, and can therefore only be known through pure thought or reason.

Gottfried Leibniz attempted to rectify what he saw as some of the problems that were not settled by Descartes by combining Descartes' work with Aristotle's notion of form and his own conception of the universe as composed of monads. He believed that ideas exist in the intellect innately, but only in a virtual sense, and it is only when the mind reflects on itself that those ideas are actualized.

Nicolas Malebranche is another well-known Rationalist, who attempted to square the Rationalism of René Descartes with his strong Christian convictions and his implicit acceptance of the teachings of St. Augustine. He posited that although humans attain knowledge through ideas rather than sensory perceptions, those ideas exist only in God, so that when we access them intellectually, we apprehend objective truth. His views were hotly contested by another Cartesian Rationalist and Jensenist Antoine Arnauld (1612 - 1694), although mainly on theological grounds.

In the 18th Century, the great French rationalists of the Enlightenment (often known as French Rationalism) include Voltaire, Jean-Jacques Rousseau and Charles de Secondat (Baron de Montesquieu) (1689 - 1755). These philosophers produced some of the most powerful and influential political and philosophical writing in Western history, and had a defining influence on the subsequent history of Western democracy and Liberalism.

Immanuel Kant started as a traditional Rationalist, having studied Leibniz and Christian Wolff (1679 - 1754) but, after also studying the empiricist David Hume's works, he developed a distinctive and very influential Rationalism of his own, which attempted to synthesize the traditional rationalist and empiricist traditions.

During the middle of the 20th Century there was a strong tradition of organized Rationalism (represented in Britain by the Rationalist Press Association, for example), which was particularly influenced by free thinkers and intellectuals. However, Rationalism in this sense has little in common with traditional Continental Rationalism, and is marked more by a reliance on empirical science. It accepted the supremacy of reason but insisted that the results be verifiable by experience and independent of all arbitrary assumptions or authority.

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