Empiricism

• 1. Introduction

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  Empiricism is the view that true knowledge, or the justification for a belief, comes only or mainly from sensory experience and empirical evidence. One ancient version of the idea survives in a single image from the doxographer Aetius, who summed up what the Stoics taught. In his words: "When a man is born, the Stoics say, he has the commanding part of his soul like a sheet of paper ready for writing upon."

  Empiricists draw a practical lesson from that picture. Humans have cognitive biases and limitations, they say, and those lead to errors of judgement when we rely on pure logical reasoning. Evidence gathered through the senses, they argue, is the more reliable route to the truth. Ideas should be formed from evidence rather than from innate ideas or inherited tradition.

  But a blank sheet raises awkward questions. If the mind starts empty, what exactly writes on it, and can that handwriting be trusted? If every belief rests on what has been experienced, what becomes of the things nobody has experienced? And how did a debate among philosophers turn into a working rule for scientists, who test every hypothesis against observation of the natural world?

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• 2. Ancient and Medieval Tablets

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  Somewhere between 600 and 200 BCE, a philosopher named Kanada founded the Vaisheshika school of Hindu philosophy. It accepted only two reliable sources of knowledge, perception and inference, a position set out in his Vaiśeṣika Sūtra. The Charvaka school took a stricter line. It held that perception alone is reliable, because inference brings knowledge only with uncertainty.

  In 330 BCE, a group of Greek medical practitioners founded the empiric school, the earliest Western proto-empiricists. They rejected the dogmatic school and preferred to rely on observing phantasiai, the appearances of things. The Pyrrhonist school of philosophy was closely allied with them and made the philosophical case for their approach.

  Around 350 BC, Aristotle wrote in On the Soul that the mind is like a writing tablet bearing no actual writing. Thought, he said, is in the mind the way letters are on such a tablet. His explanation was not strictly empiricist in the modern sense, though. It rested on his theory of potentiality and actuality, and sense experience still needed the help of the active nous. Plato had pictured the mind as something that existed in the heavens before being sent down to join a body, as in the Phaedo and the Apology. Aristotle gave sense perception a more important place than Plato did. Medieval commentators boiled his position down to a Latin slogan, nihil in intellectu nisi prius fuerit in sensu, meaning nothing in the intellect without first being in the senses.

  Islamic philosophers of the Middle Ages then developed the tablet into a fuller theory. Al Farabi began the work around the year 872. Avicenna, who lived from about 980 to 1037, built it into an elaborate system. For him the tablet is a pure potentiality that education brings into actuality. Knowledge comes from empirical familiarity with objects in the world, from which the mind abstracts universal concepts. Even so, he held that an immaterial active intellect, separate from any individual person, remains essential for understanding.

  Ibn Tufail, an Andalusian Muslim philosopher and novelist of the 12th century, turned the tablet into a story. His Arabic novel Hayy ibn Yaqdhan follows a feral child on a desert island. It traces his mind from a blank tablet to that of an adult, all in complete isolation from society. Ibn al-Nafis, an Arab theologian and physician, answered with Theologus Autodidactus in the 13th century. His novel also followed a feral child on a desert island, but his hero's mind grows through contact with society rather than apart from it.

  The same 13th century saw Thomas Aquinas bring the Aristotelian view that the senses are essential to the mind into scholasticism. Bonaventure, who lived from 1221 to 1274, was among Aquinas's strongest intellectual opponents and offered some of the strongest arguments for the Platonic idea of the mind. Ibn Tufail's novel would travel further than either of them. Edward Pococke the Younger published a Latin translation titled Philosophus Autodidactus in 1671, and it influenced John Locke's formulation of the blank tablet.

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• 3. Experience Over Authority

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  Niccolò Machiavelli and his friend Francesco Guicciardini began writing about politics and history in a new, realistic style in the late Renaissance. Machiavelli in particular was scornful of writers who judged everything against mental ideals. He urged people to study what he called the "effectual truth" instead.

  Leonardo da Vinci, who lived from 1452 to 1519, put the same attitude into a rule for his own work. He said: "If you find from your own experience that something is a fact and it contradicts what some authority has written down, then you must abandon the authority and base your reasoning on your own findings."

  The Italian philosopher Bernardino Telesio built an empirical metaphysical system with an enormous impact on later thinkers. It reached his students Antonio Persio and Sertorio Quattromani, and his contemporaries Thomas Campanella and Giordano Bruno. It also reached British philosophers such as Francis Bacon, who regarded Telesio as "the first of the moderns". Telesio's influence can be seen as well on the French philosophers René Descartes and Pierre Gassendi.

  Vincenzo Galilei, who lived from about 1520 to 1591, took the method somewhere unexpected: music. He was anti-Aristotelian and anti-clerical, he invented monody, and he was the father of Galileo. He tested how pitch relates to string tension and mass in stringed instruments, and to the volume of air in wind instruments. His 1581 Dialogo della musica antica e moderna, published in Florence, offered composers suggestions drawn from the same approach. The Italian word he used for experiment was esperimento.

  His tuning research also exposed the truth behind the myth of Pythagoras' hammers. The musical intervals came from the squares of the numbers involved, not from the numbers themselves, as people had believed. Discoveries like this showed that traditional authorities could be wrong, and a radically empirical attitude grew from them. Vincenzo passed it to his eldest son Galileo, who is arguably one of the most influential empiricists in history, and who regarded "experience and demonstration" as the indispensable core of valid rational enquiry.

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• 4. Locke's White Paper

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  In 1620, Francis Bacon became the first to advocate empiricism in England. Around 1640, René Descartes in France laid the main groundwork for the opposing rationalism. Bacon's natural philosophy also drew on the Swiss physician Paracelsus. Later in the 17th century, Thomas Hobbes and Baruch Spinoza were identified as an empiricist and a rationalist respectively. The label British empiricism is itself a retrospective one, and the formal distinction between the two camps was not drawn until Immanuel Kant, in Germany, around 1780. Kant sought to merge them.

  John Locke, who lived from 1632 to 1704, answered continental rationalism in 1689 with An Essay Concerning Human Understanding. He argued that the only knowledge humans can have is a posteriori, meaning based on experience. He is famous for the proposition that the mind is a tabula rasa, a blank tablet, or in his own phrase, white paper. Experiences from the senses are written on it as a person's life goes on.

  Locke said our ideas have two sources, sensation and reflection. Simple ideas cannot be broken down, while complex ideas combine simple ones and divide into substances, modes and relations. Simple ideas split further into primary and secondary qualities. Take an apple. Its primary qualities, such as the arrangement of its atomic structure, make it what it is. Its secondary qualities, such as colour, size and texture, are what we perceive and can vary while it remains an apple.

  For Locke, knowledge of things is the perception of ideas that agree or disagree with one another. That is very different from Descartes' quest for certainty.

  The camps were never as sealed as the labels suggest. Locke himself admitted that some knowledge, such as knowledge of God's existence, could be reached by intuition and reasoning alone. Robert Boyle, a prominent advocate of the experimental method, held that we also have innate ideas. And the main continental rationalists, Descartes, Spinoza and Leibniz, were themselves advocates of the empirical scientific method.

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• 5. Berkeley and Hume

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  The Irish Anglican bishop George Berkeley, who lived from 1685 to 1753, concluded that Locke's view opened a door that would lead to eventual atheism. He answered in 1710 with his Treatise Concerning the Principles of Human Knowledge. In it, things exist only by being perceived or by being something that does the perceiving. When humans are not around to perceive, God fills in. In his text Alciphron, Berkeley argued that any order we see in nature is the language or handwriting of God. His approach later came to be called subjective idealism.

  The Scottish philosopher David Hume, who lived from 1711 to 1776, responded to Berkeley's criticisms of Locke and pushed empiricism into deeper skepticism. He kept the empiricist principle that all knowledge derives from sense experience, but he accepted implications most philosophers found unwelcome. He objected to Locke's split of arguments into demonstrative and probable. Common usage, he suggested, calls for three groups: demonstrations, proofs and probabilities. By proofs he meant arguments from experience that leave no room for doubt or opposition.

  Hume sorted all human knowledge into two kinds, relations of ideas and matters of fact. The first covers mathematical and logical propositions, such as the one about the square of the hypotenuse equalling the sum of the squares of the two sides. The second covers contingent observations, such as the sun rising in the East. Every idea, he said, comes from an impression, which corresponds roughly to a sensation. To remember or imagine an impression is to have an idea, so ideas are faint copies of sensations.

  Then came the problem of induction. Hume pointed out that using inductive reasoning to justify inductive reasoning is circular. Nothing guarantees that the future will resemble the past. We cannot know by induction that the sun will keep rising in the East. We come to expect it because it has risen there repeatedly.

  Hume concluded that belief in an external world and belief in the self were not rationally justifiable. He said we should hold them anyway, because they rest deeply on instinct and custom. His lasting legacy was the doubt his arguments cast on induction, a doubt that many later skeptics reused.

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• 6. Phenomenalism and Its Doctors

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  Most of Hume's followers rejected his conclusion that belief in an external world has no rational justification. They argued that his own principles contained the justification, so instinct and custom were not the only support. Out of this came phenomenalism, an extreme empiricist theory anticipated by both Hume and Berkeley. It treats a physical object as a kind of construction out of our experiences. Whatever is physical is reducible to mental objects, properties and events, which is why the view is closely related to subjective idealism.

  A real physical thing, on this account, is a group of experiences with a constancy and coherence that hallucinations lack. In the mid-19th century, John Stuart Mill described matter as the "permanent possibility of sensation". Mill also went a step beyond Hume. He held that induction is necessary for all meaningful knowledge, including mathematics. As D.W. Hamlin summarized his position, mathematical truths were highly confirmed generalizations from experience, and logical and mathematical necessity was psychological. We are simply unable to conceive other possibilities. Hamlin called this perhaps the most extreme version of empiricism known, and it has not found many defenders.

  Critics raised three main problems with Mill. His account of direct experience distinguished only between actual and possible sensations, and it never said under what conditions those groups of permanent possibilities could exist. Berkeley had put God in that gap, while Mill and the phenomenalists left the question unanswered. That leaves unanswered how floor beams keep supporting a floor, or how trees keep growing, while no one is observing them. It also leaves open the unsettling possibility that the gap-filling entities are only possibilities and not actualities at all. Finally, calling mathematics another species of induction misreads it, because its results come from internally consistent deductive procedures.

  The phenomenalist phase of post-Humean empiricism ended by the 1940s. Philosophers realized that no finite set of statements about actual and possible sense data allows even one statement about a physical object to be deduced. Any translation had to speak of normal observers in normal conditions. To say a normal observer is present, the phenomenalist must say that a doctor inspecting the observer would find him normal. But that doctor must himself be a normal observer, which calls for a second doctor to inspect the first. A third doctor would then be needed for the second, and so on without end.

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• 7. Pragmatists at Harvard

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  In the 1870s, Charles Sanders Peirce and William James were both at Harvard, and their discussions produced the ideas of pragmatism. James popularized the term and gave Peirce full credit for it. Peirce later drifted away from the tangents the movement took, and he renamed what he saw as the original idea "pragmaticism". Along with its pragmatic theory of truth, the perspective combined the insights of experience-based and concept-based thinking.

  Peirce, who lived from 1839 to 1914, did much to lay the groundwork for the empirical scientific method used today. He sharply criticized Descartes' peculiar brand of rationalism, yet he did not reject rationalism outright. He agreed that rational concepts can be meaningful and that they necessarily go beyond the data of observation. In later years he stressed the concept-driven side of the debate to counterbalance the excesses of some of his colleagues, who had taken pragmatism in a strictly data-driven direction.

  One of his major contributions was to treat inductive and deductive reasoning as complementary rather than competing. Since Hume, a century earlier, the educated had mostly seen them as rivals. Peirce added a third form, abductive reasoning, and the three together underpin the scientific method today. His doctrine of fallibilism holds that the conclusions of science are always tentative. The rationality of science, he said, rests not on certainty but on its self-corrective character, since continued use of the method lets science detect and fix its own mistakes.

  In his Harvard Lectures on Pragmatism in 1903, Peirce went further and suggested that perception itself can be seen as a kind of abductive inference. The notion has been revived periodically in artificial intelligence and cognitive science, most recently in the work of Irvin Rock on indirect perception.

  Around the start of the 20th century, William James, who lived from 1842 to 1910, coined the term radical empiricism for an offshoot of his pragmatism. He argued that the directly apprehended universe needs no extraneous trans-empirical connective support. By that he meant that seeking supernatural explanations for natural phenomena adds no value. John Dewey, who lived from 1859 to 1952, modified James' pragmatism into a theory called instrumentalism. Dewey held that reality is determined by past experience, so humans adapt past experiences to run experiments and test their pragmatic value. The resulting ideas serve as instruments for further experimentation, in the physical sciences and in ethics alike.

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• 8. The Vienna Circle and After

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  Moritz Schlick, Otto Neurath and the rest of the Vienna Circle, together with A. J. Ayer, Rudolf Carnap and Hans Reichenbach, led an early 20th-century attempt called logical empiricism. It is also known as logical positivism or neopositivism. The goal was to join the strong emphasis of British empiricism on sensory experience to insights from mathematical logic developed by Gottlob Frege and Ludwig Wittgenstein.

  The neopositivists saw philosophy as the conceptual clarification of the methods and discoveries of the sciences. In the logical symbolism built by Frege, who lived from 1848 to 1925, and Bertrand Russell, who lived from 1872 to 1970, they saw a tool for rebuilding all scientific discourse in an ideal, logically perfect language. Such a language would be free of the ambiguities of everyday speech. They believed ordinary language gave rise to metaphysical pseudoproblems and other conceptual confusions.

  They combined Frege's thesis that mathematical truths are logical with the early Wittgenstein's idea that logical truths are mere linguistic tautologies. From this they derived a twofold classification of all propositions into the analytic, which is a priori, and the synthetic, which is a posteriori. On that basis they drew a sharp line between sentences that have sense and sentences that do not, called the verification principle. Any sentence that is not purely logical, or that cannot be verified, is devoid of meaning. Most metaphysical, ethical and aesthetic problems were therefore demoted to pseudoproblems.

  Before the 1930s, their extreme empiricism required every genuine synthetic assertion to reduce to direct observations or perceptions. Later, Carnap and Neurath abandoned this sort of phenomenalism. They preferred to translate sentences about physical objects into protocol sentences, of the form "X at location Y and at time T observes such and such".

  After World War II, thinkers including Nelson Goodman, W. V. Quine, Hilary Putnam, Karl Popper and Richard Rorty attacked the central theses of logical positivism. By the late 1960s, most philosophers agreed the movement had run its course, though its influence persists among analytic philosophers such as Michael Dummett and other anti-realists.

  The quarrel over the blank sheet has since moved into philosophy of mind and cognitive science, where empiricism stands against nativism, also called rationalism. Contemporary empiricists hold that the mind's innate endowment is limited to simple, general-purpose learning mechanisms. They think language acquisition, conceptual thought and social cognition can be explained without substantive innate mental content. Nativists reply that innate mental representations and specialized psychological mechanisms play a foundational role in how the mind acquires its knowledge.

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