Rationalism

• 1. Introduction

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  Rationalism is the philosophical view that reason is the chief source and test of knowledge. The philosopher Galen Strawson once captured what that promises: "you can see that it is true just lying on your couch. You don't have to get up off your couch and go outside and examine the way things are in the physical world. You don't have to do any science."

  That is a bold offer. It says some truths can be reached by thinking alone, with no help from the eye, the ear or the laboratory.

  Is the offer believable? Which truths could thought reach by itself, and where would it fall silent? Why did this confident school end up facing a rival camp that trusted experience first? And what did later thinkers find wrong with it?

  The answers run from the geometry of ancient Greece to the arguments of the European Enlightenment, and they begin with a quarrel about where knowledge comes from.

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• 2. Mind Against Senses

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  In the Enlightenment, René Descartes and John Locke became the best-known faces of a great philosophical quarrel. Descartes stressed that knowledge is mostly innate, so the intellect can grasp logical truths directly. Locke replied that knowledge is not mostly innate. He thought it is best gained by careful observation of the world outside the mind, through sensory experience.

  Rationalists pointed to principles in logic, mathematics, ethics and metaphysics that seemed so basic that denying them produces contradiction. Their trust in reason ran so high that physical proof looked unnecessary for certain truths.

  The tidy picture needs a warning. The labels rationalist and empiricist were drawn at a later period, and the philosophers involved would not have recognized them. Descartes and Locke, for instance, hold similar views about the nature of human ideas.

  Since the Enlightenment, rationalism has been tied to bringing mathematical methods into philosophy, as in the works of Descartes, Leibniz and Spinoza. It is often called continental rationalism, because it dominated the schools of continental Europe while empiricism held sway in Britain.

  Both camps sit under epistemology, and their dispute turns on justification, meaning the reasons a person has for holding a belief. Imagine that A makes a claim and B casts doubt on it. A's next move is normally to supply justification. The method A reaches for is exactly where rationalists and empiricists part ways. Of the related terms epistemologists use, the one most discussed by the early 21st century is "warrant".

  Rationalists claim reason can supply that justification, and the simplest demonstration uses nothing more than the number three.

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• 3. Intuition and Deduction

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  The number three is prime, and it is greater than two. A rationalist says you can simply see both facts, then deduce that some prime number greater than two exists, all without consulting experience. That small example captures the intuition and deduction thesis.

  The thesis says certain propositions are knowable by intuition alone, and others follow by deduction from the intuited ones. Intuition here is an immediate kind of rational insight. We simply "see" something, and the seeing gives us a warranted belief. Deduction is reasoning from general premises to a conclusion that is logically certain.

  Gottfried Wilhelm Leibniz, the prominent German philosopher, defended the thesis by arguing that the senses only ever supply individual cases. In his view, "all the instances which confirm a general truth, however numerous they may be, are not sufficient to establish the universal necessity of this same truth". Past events do not guarantee future ones. Necessary truths, such as those of arithmetic and geometry, must therefore rest on principles that do not depend on the senses.

  Even the empiricist David Hume was willing to accept this thesis when it describes the relationships among our own concepts.

  Rationalists turn a dial when they decide which subjects this method covers. Most agree that mathematics qualifies. Some add ethical truths, and some add metaphysics. The more subjects a rationalist claims, and the more strictly they treat intuition as infallible, the more radical the position becomes.

  Rationalists also differ on how strong a warrant must be. Some demand belief beyond even the slightest doubt, while others settle for belief beyond reasonable doubt.

  More contemporary rationalists admit that intuition can fail. A deceiver might cause them to intuit something false, just as a third party could cause them to perceive objects that do not exist.

  A second rationalist answer avoids the worry about faulty seeing by claiming that the knowledge was never acquired at all.

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• 4. Knowledge Already Inside

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  In Plato's Meno, a question about a geometry theorem produces a paradox that still bothers philosophers. We investigate a matter in order to learn it. Yet inquiry looks impossible. In the paradox's own words, "If we already have the knowledge, there is no place for inquiry. If we lack the knowledge, we don't know what we are seeking and cannot recognize it when we find it." And still, we do know some theorems.

  The innate knowledge thesis offers a way out. It holds that knowledge of some truths belongs to our rational nature. Experience may trigger the process that lifts this knowledge into consciousness, but experience does not supply the content. A photographer who changes the lens aperture does not create the background of a picture. The background was always there, and the adjustment simply brings it into focus.

  A sibling claim, the innate concept thesis, says that some of the concepts we use are also part of our rational nature. In Meditations on First Philosophy, Descartes sorted his ideas into three kinds. Some seemed innate, some adventitious and some invented. Understanding what a thing is, what truth is and what thought is seemed to come from his own nature. Hearing a noise or seeing the sun came from outside. Last, "sirens, hippogriffs and the like are my own invention."

  Adventitious ideas, such as the sensation of heat, arrive from outside sources, and you cannot will them away. Invented ideas, like those in mythology and fairy tales, are built from other ideas we already hold. Innate ideas, such as our idea of perfection, come from mental processes beyond anything experience can directly or indirectly provide.

  Leibniz defended innate concepts with a sculptor's image in his New Essays on Human Understanding. He rejected the blank tablet, the tabula rasa, in favor of a block of veined marble. If the veins in the stone mark out the figure of Hercules, then Hercules is in some sense innate in it, although labor is needed to uncover the veins by polishing and cutting away what hides them. Ideas and truths, he said, are innate like natural inclinations and potentialities.

  Philosophers disagree about whether the two innateness claims are really separate. Locke, the empiricist, treated them as the same, while Peter Carruthers argues they are distinct. A further rule of thumb follows: the farther a concept sits from experience, the more plausibly it can be called innate. We never experience perfect triangles but we do experience pain, so the triangle is the better candidate.

  Geometry had inspired rationalists long before Descartes or Leibniz, and the trail leads back to Pythagoras.

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• 5. Ancient Roots

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  Pythagoras, who lived from 570-495 BCE, is remembered above all for the theorem that carries his name. He is also credited with finding the mathematical link between the length of a lute string and the pitch of its note. He took such harmonies to reflect the ultimate nature of reality and summed up his outlook in three words: "All is number". That vision of a world governed by mathematical laws was later seen again by Galileo, who lived from 1564-1642. Pythagoras is also said to have been the first person to call himself a philosopher, a lover of wisdom.

  Plato, who lived from 427-347 BCE, held rational insight in the highest regard, as works such as the Republic show. His Theory of Forms claims that the most fundamental reality is not the changing material world we sense. It is an abstract, non-material world of forms, and only reason can reach it. According to tradition, Plato admired geometry so much that he had the words "Let no one ignorant of geometry enter" inscribed above the door of his academy.

  Aristotle, who lived from 384-322 BCE, contributed the syllogism. He defined it as a discourse in which, once certain things are supposed, something different follows of necessity. In his Prior Analytics, he confined himself to categorical syllogisms built from three categorical propositions, including a modal variety.

  The three Greeks disagreed on specific points, yet they shared a conviction that reason can expose self-evident knowledge people could not otherwise reach.

  After Aristotle's death, Western rationalist thought was often applied to theology. Augustine, the Islamic philosophers Avicenna (Ibn Sina) and Averroes (Ibn Rushd), and the Jewish theologian Maimonides all worked in this vein. The Waldensian sect folded rationalism into its movement. In the thirteenth century, Thomas Aquinas tried to merge Greek rationalism with Christian revelation.

  The Roman Catholic Church nonetheless tended to see rationalists as a threat. It described them as people who, while admitting revelation, "reject from the word of God whatever, in their private judgment, is inconsistent with human reason."

  That judgment would only sharpen with the arrival of Descartes, who lived from 1596-1650 and was later dubbed the Father of Modern Philosophy.

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• 6. The Seventeenth-Century Architects

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  Descartes believed reason alone could secure only the eternal truths. These included the truths of mathematics and the foundations of the sciences. Physics, by contrast, needed experience of the world, aided by the scientific method. His method for reaching the first kind of truth broke problems into elements that intuition can grasp, then reasoned deductively toward clear conclusions about reality.

  Dreams gave him a lever for doubt. They seem as real as sense experience, yet they cannot give knowledge. Since the senses can also produce illusions, he concluded that a rational search for truth should doubt every belief about the sensory world. He set out these ideas in Discourse on the Method and Principles of Philosophy.

  From this came his famous dictum, cogito ergo sum, or "I think, therefore I am". It is reached a priori, before any experience on the matter. Doubting your existence proves that an "I" exists to do the doubting, and he treated this as an irrefutable base for all other knowledge.

  Descartes also posited a dualism between the body, which he called "res extensa", and the mind or soul, which he called "res cogitans". Because the two substances were independent and irreducible, the link between them stayed unresolved and became the mind-body problem.

  Baruch Spinoza, who lived from 1632-1677, answered with a system built like a proof. He drew on Descartes, Euclid, Thomas Hobbes and theologians of the Jewish philosophical tradition such as Maimonides, yet he departed from the Judeo-Christian-Islamic tradition in many respects. He proposed that "God exists only philosophically." His masterwork, Ethics, has a forbidding structure modeled on Euclid's geometry and contains obscurities no one has settled. Goethe admitted that the geometrical method confused him. Still, Albert Einstein was among the believers Spinoza attracted, and his principles about the emotions carry implications for modern psychology.

  Gottfried Leibniz, who lived from 1646-1716, was the last major rationalist of the seventeenth century. He also worked in physics, logic, mathematics, jurisprudence and the philosophy of religion, and is considered one of the last universal geniuses. Rejecting both Cartesian dualism and the existence of a material world, he proposed infinitely many simple substances called monads, a term he derived from Proclus. Monads make up everything, living or not, and stand outside causality and space. To explain why the world still appears causal, he introduced his principle of pre-established harmony.

  Spinoza and Leibniz both held that all knowledge, scientific knowledge included, could be had by reason alone in principle. They also agreed that human beings cannot manage this in practice outside areas like mathematics. Leibniz admitted as much in his book Monadology, writing that "we are all mere Empirics in three fourths of our actions."

  One later German thinker, born in 1724, would take that admission seriously and turn it against both camps.

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• 7. Kant's Verdict

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  Immanuel Kant, who lived from 1724-1804, argued that human perception structures natural laws and that reason is the source of morality. His influence reaches metaphysics, epistemology, ethics, political philosophy and aesthetics. He gave his own epistemology the name Transcendental Idealism and first laid it out in The Critique of Pure Reason.

  In that book Kant found fault with both the rationalist and the empiricist creed. To the rationalists he said pure reason goes wrong when it pushes past its limits. It then claims to know matters that lie beyond any possible experience: the existence of God, free will and the immortality of the human soul. Kant called such objects "The Thing in Itself". Because they lie beyond all possible experience, he argued, we cannot know them.

  To the empiricists he granted that experience is essential for human knowledge. But he insisted that reason is needed to turn experience into coherent thought. Both reason and experience are therefore required.

  He also denied that thought is mere analysis. As one summary of his view puts it, "a priori concepts do exist, but if they are to lead to the amplification of knowledge, they must be brought into relation with empirical data".

  That middle path is one reason Kant is described as setting the terms every later thinker has had to grapple with.

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• 8. The Word's Afterlife

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  In 1670, a writer named Sanderson dismissed someone as "a mere rationalist, that is to say in plain English an atheist of the late edition..." The philosopher John Cottingham cites the remark to show how the word once worked. In the 17th and 18th centuries, "rationalist" often labeled anti-clerical, anti-religious free thinkers, and for a time it was plainly pejorative. A method had become socially tangled up with atheism, a worldview. Today, terms like humanist or materialist have largely taken its place.

  The American psychologist William James criticized rationalism for being out of touch with reality. He also objected that it pictures the universe as a closed system, whereas he saw it as open.

  Proponents of emotional choice theory attack from another side. Rationalism, they say, assumes decisions come from conscious, reflective calculation. Neuroscience suggests only a small part of the brain's activity runs at the level of conscious reflection, and most of the rest consists of unconscious appraisals and emotions. They add that emotions cannot be separated from the social environment, with its norms and identities, so standard rationalist models struggle to include them.

  Among professional philosophers, rationalism has become a rarer label. Many specialized versions appear instead. Robert Brandom took up "rationalist expressivism" and "rationalist pragmatism" for parts of his program in Articulating Reasons. He also identified "linguistic rationalism" as a key thesis of Wilfrid Sellars, the claim that the contents of propositions are essentially what can serve as both premises and conclusions of inferences.

  Outside the academy, participants in the internet communities around LessWrong and Slate Star Codex have called themselves rationalists. They mean rationality, not the philosophy of Descartes. Critics such as Timnit Gebru use the label in the same way.

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