Rationalism

Rationalism (Latin ratio, “reason”), in philosophy, a system of thought that emphasizes the role of reason in obtaining knowledge, in contrast to empiricism, which emphasizes the role of experience, especially sense perception.

Rationalism has appeared in some form in nearly every stage of Western philosophy, but it is primarily identified with the tradition stemming from the 17th-century French philosopher, mathematician, and scientist René Descartes. Descartes believed that geometry represented the ideal for all sciences and philosophy. He held that by means of reason alone, certain universal, self-evident truths could be discovered, from which much of the remaining content of philosophy and the sciences could be deductively derived. He assumed that these self-evident truths were innate, not derived from sense experience. This type of rationalism was developed by other European thinkers, such as the Dutch philosopher Baruch Spinoza, who wrote his Ethics (1677) in the form of a geometrical treatise, and the German philosopher and mathematician Gottfried Wilhelm Leibniz.

According to the traditional division of the subject, rationalism was opposed by the British empiricists—John Locke, George Berkeley, and David Hume—who believed that all ideas are derived from sense experience. This traditional division goes back to Immanuel Kant, and has recently come under challenge, as overstating the differences between the two sets of philosophers. Book 4 of Locke’s Essay Concerning Human Understanding (1690), for example, expounds a model of scientific knowledge that clearly owes much to Descartes.

Nor, contrary to popular tradition, did the rationalists maintain that sensory experience is irrelevant to knowledge. They thought that the senses provided some test of the principles and postulates of reason. Nevertheless, the rationalists regarded the senses as in themselves a source of mere opinion, rather than of true knowledge. They sought knowledge that was systematic and interconnected. Sense experience could at best establish the regular association of certain phenomena; it could not explain why it should happen, though, or show that it had to happen. The rationalists held that truly scientific knowledge would overcome these deficiencies.

The claim of empiricist philosophers to derive all ideas from experience was challenged by Leibniz, who maintained that, for example, the concept of necessity could not come from that source. Far from being found in experience, that which is necessary holds true regardless of what experiences occur. Whereas the empiricists thought of the human mind as a blank tablet (tabula rasa), receptive of and shaped by experiences, the rationalists thought of the mind as predisposed towards certain concepts: experience might be necessary to stimulate those concepts, but they were not to be read off from experience. In this sense the rationalists held them to be innate. Thus, for example, they held that substance and quality are categories into which sensory experience is organized, rather than items to be found in experience. A similar position held for identity. Descartes maintained that as we observe a piece of wax being heated by a fire, it is clear that all the properties given to the senses alter (for example, it becomes soft instead of hard, colourless instead of white). Yet, despite the evidence of the senses, it is clear that it is the same piece of wax that undergoes and survives these changes. Descartes thought that this could only be known through intellectual intuition.

Despite having doctrines in common, the rationalists differed among themselves, particularly in their accounts of substance and the mind. Descartes thought that there were two sorts of substance, mind and body, each capable of existing apart from the other but united in human beings. Spinoza argued that the mental and the physical were two aspects of the same reality, and that that reality consists of one substance, of which human beings and all living things are modes (or modifications). Leibniz believed that fundamentally there exist infinitely many indivisible monads, of which everything was constituted, all in some sense perceiving and striving, though not all fully possessed of consciousness. The rationalists all agreed that a substance had independent existence; Spinoza took the step of identifying his one substance as God.

The question of how there could be free actions in a world in which effects followed necessarily from causes was another issue that taxed the rationalists. Descartes thought that there was freedom in the realm of the mental and necessity in the realm of the physical, but that everything a person does follows necessarily from his or her complete notion, and that God chose to create a world in which that person’s complete notion was instantiated. Spinoza maintained that free actions are those determined by the self alone.

The rationalists were keenly interested in science and played an important part in its development; not so much by any discoveries they made as by their willingness to press the importance of the mathematical and geometrical approach in going beyond, and helping to explain, sensory appearances.

Epistemological rationalism has been applied to other fields of philosophical inquiry. Rationalism in ethics is the claim that certain primary moral ideas are innate in humankind and that such first moral principles are self-evident to the rational faculty. Rationalism in the philosophy of religion is the claim that the fundamental principles of religion are innate or self-evident and that revelation is not necessary, as in deism. Since the end of the 1800s, however, rationalism has chiefly played an anti-religious role in theology.