Rationalism is a broad family of positions in epistemology. Perhaps the best general description of rationalism is the view that there are some distinctive aspects or faculties of the mind that (1) are distinct from passive aspects of the mind such as sense-perceptions and (2) someway or other constitute a special source (perhaps only a partial source) of knowledge. These distinctive aspects are typically associated or identified with human abilities to engage in mathematics and abstract reasoning, and the knowledge they provide is often seen as of a type that could not have come from other sources. Philosophers who resist rationalism are usually grouped under the heading of empiricists, who are often allied under the claim that all human knowledge comes from experience.
The debate around which the rationalism/empiricism distinction revolves is one of the oldest and most continuous in philosophy. Some of Plato's most explicit arguments address the topic and it was arguably the central concern of many of the Modern thinkers. Indeed, Kant's principal works were concerned with "pure" faculties of reason. Contemporary philosophers have advanced and refined the issue, though there are current thinkers who align themselves with either side of the tradition.
It is difficult to identify a major figure in the history to whom some rationalist doctrine has not been attributed at some point. One reason for this is that there is no question that humans possess some sort of reasoning ability that allows them to come to know some facts they otherwise wouldn't (for instance, mathematical facts), and every philosopher has had to acknowledge this fact. Another reason is that the very business of philosophy is to achieve knowledge by using the rational faculties, in contrast to, for instance, mystical approaches to knowledge. Nevertheless, some philosophical figures stand out as attributing even greater significance to reasoning abilities. Three are discussed here: Plato, Descartes, and Kant.
The most famous metaphysical doctrine of the great Greek philosopher Plato is his doctrine of "Forms," as espoused in The Republic and other dialogues. The Forms are described as being outside of the world as experience by the senses, but as somehow constituting the metaphysical basis of the world. Exactly how they fulfill this function is generally only gestured at through analogies, though the Timaeus describes the Forms as operating as blueprints for the craftsman of the universe.
The distinctiveness of Plato's rationalism lies in another aspect of his theory of Forms. Though the common sense position is that the senses are one's best means of getting in touch with reality, Plato held that human reasoning ability was the one thing that allowed people to approach the Forms, the most fundamental aspects of reality. It is worth pausing to reflect on how radical this idea is: On such a view, philosophical attempts to understand the nature of "good" or "just" are not mere analyses of concepts formed, but rather explorations of eternal things that are responsible for shaping the reality of the sensory world.
The French philosopher René Descartes, whose Meditations on First Philosophy defined the course of much philosophy from then up till the present day, stood near the beginning of the Western European Enlightenment. Impressed by the power of mathematics and the development of the new science, Descartes was confronted with two questions: How was it that people were coming to attain such deep knowledge of the workings of the universe, and how was it that they had spent so long not doing so?
Regarding the latter question, Descartes concluded that people had been mislead by putting too much faith in the testimony of their senses. In particular, he thought such a mistake was behind the then-dominant physics of Aristotle. Aristotle and the later Scholastics, in Descartes' mind, had used their reasoning abilities well enough on the basis of what their senses told them. The problem was that they had chosen the wrong starting point for their inquiries.
By contrast, the advancements in the new science (some of which Descartes could claim for himself) were based in a very different starting point: The "pure light of reason." In Descartes' view, God had equipped humans with a faculty that was able to understand the fundamental essence of the two types of substance that made up the world: Intellectual substance (of which minds are instances) and physical substance (matter). Not only did God give people such a faculty, Descartes claimed, but he made them such that, when using the faculty, they are unable to question its deliverances. Not only that, but God left humanity the means to conclude that the faculty was a gift from a non-deceptive omnipotent creator.
In some respects, the German philosophy Immanuel Kant is the paradigm of an anti-rationalist philosopher. A major portion of his central work, the 1781 Critique of Pure Reason, is specifically devoted to attacking rationalist claims to have insight through reason alone into the nature of the soul, the spatiotemporal/causal structure of the universe, and the existence of God. Plato and Descartes are among his most obvious targets.
For instance, in his evaluation of rationalist claims concerning the nature of the soul (the chapter of the Critique entitled "The Paralogisms of Pure Reason"), Kant attempts to diagnose how a philosopher like Descartes could have been tempted into thinking that he could accomplish deep insight into his own nature by thought alone. One of Descartes' conclusions was that his mind, unlike his body, was utterly simple and so lacked parts. Kant claimed that Descartes mistook a simple experience (the thought, "I think") for an experience of simplicity. In other words, he saw Descartes as introspecting, being unable to find any divisions within himself, and thereby concluding that he lacked any such divisions and so was simple. But the reason he was unable to find divisions, in Kant's view, was that by mere thought alone we are unable to find anything.
At the same time, however, Kant was an uncompromising advocate of some key rationalist intuitions. Confronted with the Scottish philosopher David Hume's claim that the concept of "cause" was merely one of the constant conjunction of resembling entities, Kant insisted that all Hume really accomplished was in proving that the concept of causation could not possibly have its origin in human senses. What the senses cannot provide, Kant claimed, is any notion of necessity, yet a crucial part of our concept of causation is that it is the necessary connection of two entities or events. Kant's conclusion was that this concept, and others like it, must be a precondition of sensory experience itself.
In his moral philosophy (most famously expounded in his Groundwork for the Metaphysics of Morals), Kant made an even more original claim on behalf of reason. The sensory world, in his view, was merely ideal, in that the spatiotemporal/sensory features of the objects people experience have their being only in humanity's representations, and so are not features of the objects in themselves. But this means that most everyday concepts are simply inadequate for forming any notion whatsoever of what the world is like apart from our subjective features. By contrast, Kant claimed that there was no parallel reason for thinking that objects in themselves (which include our soul) do not conform to the most basic concepts of our higher faculties. So while those faculties are unable to provide any sort of direct, reliable access to the basic features of reality as envisioned by Plato and Descartes, they and they alone give one the means to at least contemplate what true reality might be like.
In the early part of the twentieth century, a philosophical movement known as Logical Positivism set the ground for a new debate over rationalism. The positivists (whose ranks included Otto Neurath and Rudolf Carnap) claimed that the only meaningful claims were those that could potentially be verified by some set of experiential observations. Their aim was to do away with intellectual traditions that they saw as simply vacuous, including theology and the majority of philosophy, in contrast with science.
As it turned out, the Positivists were unable to explain how all scientific claims were verifiable by experience, thus losing their key motivation (for instance, no set of experiences could verify that all stars are hot, since no set of experiential observations could itself confirm that one had observed all the stars). Nevertheless, their vision retained enough force that later philosophers felt hard-pressed to explain what, if anything, was epistemically distinctive about the non-sensory faculties. One recent defense of rationalism can be found in the work of contemporary philosophers such as Laurence Bonjour (the recent developments of the position are, in general, too subtle to be adequately addressed here). Yet the charge was also met by a number of thinkers working in areas as closely related to psychology as to philosophy.
A number of thinkers have argued for something like Kant's view that people have concepts independently of experience. Indeed, the groundbreaking work of the linguist Noam Chomsky (which he occasionally tied to Descartes) is largely based on the assumption that there is a "universal grammar"—that is, some basic set of linguistic categories and abilities that necessarily underlie all human languages. One task of linguistics, in Chomsky's view, is to look at a diversity of languages in order to determine what the innate linguistic categories and capacities are.
A similar proposal concerning human beliefs about mentality itself has been advanced by Peter Carruthers. One intuitive view is that each of us comes to attribute mental states to other people only after a long developmental process where people learn to associate observable phenomena with their own mental states, and thereby with others. Yet, Carruthers argues, this view simply cannot account for the speed and complexity of humans' understanding of others' psychology at very early ages. The only explanation is that some understanding of mentality is "hard-wired" in the human brain.
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