Rationalism is a philosophy in which a high regard is given to reason (specifically logic) and to empirical observation.
From the strict philosophical standpoint, rationalism is the view that all or most truth is deductive and a priori, deriving logically from a set of axioms gained by intuition or inherent knowledge (and not from studying the world around us empirically). However, the term is not very often used so strictly, so this form of rationalism is generally known in English-speaking philosophy as continental rationalism, as its original proponents, such as Ren Descartes, were largely situated in continental Europe.
The term is more commonly used to refer to a synthesis of continental rationalism with its former rival philosophy, empiricism. This looser rationalism holds that empirical observation is more useful than intuition for gaining one’s starting axioms, but one can use deductive reasoning from these axioms just as well. The best embodiment of this way of gaining knowledge is the scientific method; hence, rationalists tend to give high regard to science, designating it as the primary or sole proper source of truth.
RationalWiki is devoted to this sort of rational analysis of empirical evidence to form conclusions; most RationalWiki editors are very skeptical of other ways of knowing.
The idea of being “rational” is distinct and broader than the philosophy of rationalism. To be “rational” is synonymous with a “sane” or “functional” way of thinking. If one is “rational,” then in common parlance this means that one can think clearly and is capable of intelligently assessing new ideas when presented.
The opposite term, “irrational,” is used to signify someone who cannot or will not think clearly. If a thought or action is irrational, it signifies something that is not just incorrect, but perverse, insane, or beneath consideration.
Rationalism was first formulated in classical times by philosophers such as Socrates and Plato. Many of the Socratic dialogues would use a conversational process to work out logical inconsistencies in ideas that were held by contemporaries to be “common sense,” such as the definition of “the good.” In this historical sense Rationalism was distinct and separate from Empiricism (see below), as these early Rationalists didn’t deem it necessary to use observation – in the modern use, Rationalists who would combine both the logical reasoning of Rationalism with the observational checks of Empiricism.
But at the time, virtually everyone even the great philosophers believed that various things were known by people inherently. Aside from a few schools of thought which suggested that nothing could ever be known as true (pyrrhonism), few thought to discard a priori beliefs and start from scratch with only that which was known to be true. Thus, at this point historical Rationalism closely resembled the way philosophers still define the philosophy.
The 16th century philosopher Descartes, however, attempted to create a whole philosophy through pure reason in his Discourse on Method and its succeeding works: he began with the only thing of which he thought he could be certain, that there was an “I” that was thinking – often rendered in the Latin of cogito ergo sum (“I think, therefore I am.”). His process ushered in a new era in rationalism, concurrent with the greater Enlightenment. At that time the philosophy began to resemble modern empiricism more than its own ancient ancestor, especially during the era of Romanticism when Enlightenment ideas were challenged and sensory perception was given more of a hearing.
Loose use in the time since has led to the fuzzy state of the term today, particularly when combined with the similar but much broader notion of being rational.
Rationalism is a term of art in psychology referring to the school of thought that sees certain elements of cognition as innate. (For this reason, it is sometimes used synonymously with the terms “innatism” or “nativism.”) Rationalism in psychology is identified with the philosophical tradition of the same name. During the 20th century, Noam Chomsky became associated with rationalism due to his positing the concept of an innate “language acquisition device.”
Rationalism, or “economic rationalism,” is also a term of art in economics. It is generally used today in Australia to refer to the local brand of neoliberal economic and political policy, though it was also used by scholars such as Max Weber in reference to the Protestant work ethic.
Another straightforward conception of rationality is that an individual acts rationally if they act in the way that, on reflection, they believe best suits achievement of their aims. This conception, naturally, gives rise to the common conception when on reflection it is believed that the aim of truth can best be achieved through factual analysis and the scientific method.
This approach, however, is problematic, as it denies the existence of any sort of objective logic independent of human perception. Many, on reflection, believe that astrology, Scientology, homeopathy and other ridiculous nonsense best suits achievement of their aims. If these people are to be held to be irrational, a new criterion must be put in the place of on reflection. Usually the criterion is modified such that the rational person must reasonably believe they have a methodology to achieve their aims, leaving question of what it means to reasonably believe that a methodology will achieve certain aims.
 Alvin Plantinga’s concept of rationalism neatly distinguishes reason from “raving madness” by conceiving of reason as “not raving mad”. Of course Plantinga has to give us a good idea of what is not “raving mad”, which he does with his concept of “proper function”. Just as a clock, functioning properly, is a reliable indicator of the time, human senses, functioning properly, are reliable indicators of the world. Acting in accordance with the proper function of our faculties is rational.
The problem of what constitutes function, and proper function at that, is more than a question of what our faculties do (a fast clock tells the incorrect time, but this is not its function). Both function and proper function have an element of what things should be doing. As should is a difficult concept to introduce in a mechanistic description of how things happen to be, Plantinga sources the “should”, the “purpose” of our faculties in a concept of God.
Critical rationalism ( Karl Popper) differentiates from the above conceptions of rationality by rejecting any positive content in reason. Reason, critical rationalism holds, does not provide ‘reasons’: it does not give positive recommendations about what beliefs should be held. Reason operates negatively, restricting the beliefs that can be held. It does this through criticism, subjecting pre-adopted beliefs to tests in an effort to refute them.
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