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Rationalism,in Western philosophy, the view that regards reason as the chief source and test of knowledge. Holding that reality itself has an inherently logical structure, the rationalist asserts that a class of truths exists that the intellect can grasp directly. There are, according to the rationalists, certain rational principlesespecially in logic and mathematics, and even in ethics and metaphysicsthat are so fundamental that to deny them is to fall into contradiction. The rationalists confidence in reason and proof tends, therefore, to detract from their respect for other ways of knowing.

Rationalism has long been the rival of empiricism, the doctrine that all knowledge comes from, and must be tested by, sense experience. As against this doctrine, rationalism holds reason to be a faculty that can lay hold of truths beyond the reach of sense perception, both in certainty and generality. In stressing the existence of a natural light, rationalism has also been the rival of systems claiming esoteric knowledge, whether from mystical experience, revelation, or intuition, and has been opposed to various irrationalisms that tend to stress the biological, the emotional or volitional, the unconscious, or the existential at the expense of the rational.

Rationalism has somewhat different meanings in different fields, depending upon the kind of theory to which it is opposed.

In the psychology of perception, for example, rationalism is in a sense opposed to the genetic psychology of the Swiss scholar Jean Piaget (18961980), who, exploring the development of thought and behaviour in the infant, argued that the categories of the mind develop only through the infants experience in concourse with the world. Similarly, rationalism is opposed to transactionalism, a point of view in psychology according to which human perceptual skills are achievements, accomplished through actions performed in response to an active environment. On this view, the experimental claim is made that perception is conditioned by probability judgments formed on the basis of earlier actions performed in similar situations. As a corrective to these sweeping claims, the rationalist defends a nativism, which holds that certain perceptual and conceptual capacities are innateas suggested in the case of depth perception by experiments with the visual cliff, which, though platformed over with firm glass, the infant perceives as hazardousthough these native capacities may at times lie dormant until the appropriate conditions for their emergence arise.

Chomsky, NoamAPIn the comparative study of languages, a similar nativism was developed in the 1950s by the innovating syntactician Noam Chomsky, who, acknowledging a debt to Ren Descartes (15961650), explicitly accepted the rationalistic doctrine of innate ideas. Though the thousands of languages spoken in the world differ greatly in sounds and symbols, they sufficiently resemble each other in syntax to suggest that there is a schema of universal grammar determined by innate presettings in the human mind itself. These presettings, which have their basis in the brain, set the pattern for all experience, fix the rules for the formation of meaningful sentences, and explain why languages are readily translatable into one another. It should be added that what rationalists have held about innate ideas is not that some ideas are full-fledged at birth but only that the grasp of certain connections and self-evident principles, when it comes, is due to inborn powers of insight rather than to learning by experience.

Common to all forms of speculative rationalism is the belief that the world is a rationally ordered whole, the parts of which are linked by logical necessity and the structure of which is therefore intelligible. Thus, in metaphysics it is opposed to the view that reality is a disjointed aggregate of incoherent bits and is thus opaque to reason. In particular, it is opposed to the logical atomisms of such thinkers as David Hume (171176) and the early Ludwig Wittgenstein (18891951), who held that facts are so disconnected that any fact might well have been different from what it is without entailing a change in any other fact. Rationalists have differed, however, with regard to the closeness and completeness with which the facts are bound together. At the lowest level, they have all believed that the law of contradiction A and not-A cannot coexist holds for the real world, which means that every truth is consistent with every other; at the highest level, they have held that all facts go beyond consistency to a positive coherence; i.e., they are so bound up with each other that none could be different without all being different.

In the field where its claims are clearestin epistemology, or theory of knowledgerationalism holds that at least some human knowledge is gained through a priori (prior to experience), or rational, insight as distinct from sense experience, which too often provides a confused and merely tentative approach. In the debate between empiricism and rationalism, empiricists hold the simpler and more sweeping position, the Humean claim that all knowledge of fact stems from perception. Rationalists, on the contrary, urge that some, though not all, knowledge arises through direct apprehension by the intellect. What the intellectual faculty apprehends is objects that transcend sense experienceuniversals and their relations. A universal is an abstraction, a characteristic that may reappear in various instances: the number three, for example, or the triangularity that all triangles have in common. Though these cannot be seen, heard, or felt, rationalists point out that humans can plainly think about them and about their relations. This kind of knowledge, which includes the whole of logic and mathematics as well as fragmentary insights in many other fields, is, in the rationalist view, the most important and certain knowledge that the mind can achieve. Such a priori knowledge is both necessary (i.e., it cannot be conceived as otherwise) and universal, in the sense that it admits of no exceptions. In the critical philosophy of Immanuel Kant (17241804), epistemological rationalism finds expression in the claim that the mind imposes its own inherent categories or forms upon incipient experience (see below Epistemological rationalism in modern philosophies).

In ethics, rationalism holds the position that reason, rather than feeling, custom, or authority, is the ultimate court of appeal in judging good and bad, right and wrong. Among major thinkers, the most notable representative of rational ethics is Kant, who held that the way to judge an act is to check its self-consistency as apprehended by the intellect: to note, first, what it is essentially, or in principlea lie, for example, or a theftand then to ask if one can consistently will that the principle be made universal. Is theft, then, right? The answer must be No, because, if theft were generally approved, peoples property would not be their own as opposed to anyone elses, and theft would then become meaningless; the notion, if universalized, would thus destroy itself, as reason by itself is sufficient to show.

In religion, rationalism commonly means that all human knowledge comes through the use of natural faculties, without the aid of supernatural revelation. Reason is here used in a broader sense, referring to human cognitive powers generally, as opposed to supernatural grace or faiththough it is also in sharp contrast to so-called existential approaches to truth. Reason, for the rationalist, thus stands opposed to many of the religions of the world, including Christianity, which have held that the divine has revealed itself through inspired persons or writings and which have required, at times, that its claims be accepted as infallible, even when they do not accord with natural knowledge. Religious rationalists hold, on the other hand, that if the clear insights of human reason must be set aside in favour of alleged revelation, then human thought is everywhere rendered suspecteven in the reasonings of the theologians themselves. There cannot be two ultimately different ways of warranting truth, they assert; hence rationalism urges that reason, with its standard of consistency, must be the final court of appeal. Religious rationalism can reflect either a traditional piety, when endeavouring to display the alleged sweet reasonableness of religion, or an antiauthoritarian temper, when aiming to supplant religion with the goddess of reason.

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rationalism | Britannica.com