Rationalism vs Empiricism: How Did They Differ?
Rationalism and empiricism were the two principal philosophies of nature in the Scientific Revolution. Descartes, Leibniz, Bacon, Locke, and Newton represented the two sides.
Rationalism vs Empiricism: How Did They Differ?
The two principal philosophical orientations of the Scientific Revolution were rationalism and empiricism. The rationalists — most importantly René Descartes, Gottfried Wilhelm Leibniz, and Baruch Spinoza — held that certain knowledge could be obtained by reason alone, independently of experience. The empiricists — most importantly Francis Bacon, John Locke, and Isaac Newton — held that all knowledge of matters of fact is ultimately derived from sensory experience. The two traditions defined the agenda of early-modern philosophy of science, and the contrast between them remained central to philosophical debate for the next three centuries.
The Core of the Distinction
The rationalist position, in its strongest form, is that there are truths about the world that can be known by reason alone, without any appeal to experience. The clearest example is mathematics: the truths of geometry and arithmetic are known to be true simply by thinking about them, and the evidence of the senses is not required. The empiricist position, in its strongest form, is that there are no such truths about the world. All knowledge of matters of fact — including, in the strongest forms of empiricism, knowledge of mathematics itself — is derived from experience. In practice, the philosophers of the Scientific Revolution did not hold these positions in their strongest forms. Most were modest rationalists or modest empiricists, accepting some mixture of the two. Descartes, the great rationalist, was a careful experimenter in physics. Newton, the great empiricist, was a sophisticated mathematician. The contrast between them is real, but it is a contrast of emphasis rather than of absolute commitment.
Descartes and the Rationalist Tradition
René Descartes (1596–1650) is the most important figure in the rationalist tradition. In his Discourse on Method (1637) and Meditations on First Philosophy (1641), Descartes proposed a program of methodical doubt — a systematic doubt of all beliefs that admit of any doubt — and the construction of a new philosophy on the secure foundation of the cogito (the famous “I think, therefore I am”) and the clear and distinct ideas of mathematics. Descartes sought to derive the laws of nature from the clear and distinct idea of extension — the property of bodies in space — and to explain the entire physical world as a system of corpuscles in motion governed by mechanical laws. He was a careful experimenter in physics — his work on optics, meteorology, and physiology is full of carefully designed experiments — but he held that the ultimate justification of his natural philosophy was rational, not empirical. The rationalist tradition was continued by Gottfried Wilhelm Leibniz (1646–1716), the German mathematician and philosopher, who developed an elaborate metaphysical system in which the world is composed of monads — simple substances that do not interact causally but are synchronized by a pre-established harmony established by God. Leibniz also invented the calculus, independently of Newton, and his work on the foundations of the calculus had an enormous influence on the growth of mathematics.
Bacon and the Empiricist Tradition
Francis Bacon (1561–1626) is the most important figure in the empiricist tradition, at least in the English-speaking world. In his Novum Organum (1620), Bacon criticized the Aristotelian-Scholastic logic and proposed a new method based on the gradual ascent from particular observations to general axioms. Bacon was not a complete empiricist — he believed, for example, that the existence of God could be known by reason — but he was an empiricist about natural philosophy. The proper method of investigating nature, in his view, was the careful collection of instances, the tabulation of natural phenomena, and the gradual elimination of false generalizations. The Aristotelian tradition, by contrast, was too ready to construct theories on the basis of inadequate observation, and too confident in the conclusions drawn by deductive reasoning from those theories. The empiricist tradition was continued in the work of John Locke (1632–1704), whose Essay Concerning Human Understanding (1690) argued that the mind is, at birth, a tabula rasa — a blank slate — and that all of its ideas are derived from experience. Locke’s empiricism had an enormous influence on the growth of modern philosophy, especially in the English-speaking world. The Scottish philosopher David Hume (1711–1776) carried the empiricist critique even further, arguing that the principle of causation itself — the foundation of all natural philosophy — is not rationally justified, but is a habit of mind derived from constant conjunction in experience.
Mathematics and Empiricism
A crucial question in the debate between rationalists and empiricists is the status of mathematics. The rationalists held that mathematical truths are known by reason alone, and that the rational structure of mathematics is the model for all genuine knowledge. The empiricists had to explain how mathematical knowledge, which seemed so obviously non-empirical, could be derived from experience. The strongest form of empiricism, that of David Hume, treats even mathematical knowledge as a habit of mind derived from experience. The most modest form, that of Francis Bacon, allows that mathematical truths are known by reason, but holds that natural-philosophical knowledge must be grounded in observation and experiment. Most natural philosophers of the Scientific Revolution — including Newton — were empiricists in this modest sense, allowing the rational character of mathematics while insisting on the empirical grounding of natural philosophy.
The Methodological Turn: Newton
The most important philosophical move in the late seventeenth century was Isaac Newton’s synthesis of the rationalist and empiricist traditions. Newton’s method, as articulated in the Principia (1687) and the Opticks (1704), was neither pure deduction from first principles nor naive induction from observations. It was a combination of the two: mathematical deduction from empirically established laws, and empirical confirmation of the mathematical consequences of those laws. The fourth of Newton’s Rules of Reasoning in Philosophy captures the synthesis: “In experimental philosophy we are to look upon propositions inferred by general induction from phenomena as accurately or very nearly true, notwithstanding any contrary hypotheses that may be imagined, till such time as other phenomena occur, by which they may either be made more accurate, or liable to exceptions.” This was an important philosophical move. It acknowledged that science must be grounded in phenomena (the empirical commitment), while also allowing that the propositions of natural philosophy could be taken as (provisionally) true on the basis of their agreement with phenomena. Newton’s method, in this sense, was a form of empirical realism — a commitment to the existence of a world that scientific theories describe, combined with a careful, fallibilist method for investigating that world. The story of Newton’s method is told in the article on What Is the Scientific Method? and in the page on Bacon and Descartes.
Innate Ideas
A specific issue that divided the rationalists and the empiricists was the doctrine of innate ideas. The rationalists, following Descartes, held that certain ideas — including the ideas of God, of substance, and of mathematical truths — are innate, imprinted on the mind from birth and not derived from experience. The empiricists, following Locke, denied that there are any innate ideas; the mind, at birth, is a blank slate, and all of its ideas are derived from experience. The dispute about innate ideas was an important part of the philosophical landscape of the seventeenth century, and it had practical consequences for the new natural philosophy. If there are innate ideas, then the rationalist philosopher can appeal to them in the construction of natural philosophy; if there are not, then the empirical philosopher must rely on observation and experiment to establish the foundations of natural philosophy. The story of these debates is part of the page on Bacon and Descartes.
The Source of Knowledge
The deepest question dividing the two traditions is the source of human knowledge. The rationalists held that reason is the ultimate source — that there are truths about the world that can be known simply by thinking about them. The empiricists held that experience is the ultimate source — that all knowledge of matters of fact is derived from the senses. The two positions are not mutually exclusive in their modest forms. A modest rationalist can accept that natural knowledge must be grounded in experience while holding that the rational structure of mathematics reveals truths that are not themselves empirical. A modest empiricist can accept that mathematics is known by reason while holding that natural philosophy must be grounded in experience. The two traditions have, in this sense, more in common than is often appreciated, and the most important natural philosophers of the Scientific Revolution — Newton most conspicuously — found ways to combine them.
The Debate in the Eighteenth and Nineteenth Centuries
The rationalist-empiricist debate continued into the eighteenth and nineteenth centuries. The great German philosopher Immanuel Kant (1724–1804) attempted a synthesis in his Critique of Pure Reason (1781), arguing that knowledge arises from the joint contribution of sense and understanding — that neither pure reason nor pure experience can account for our knowledge of the world. Kant’s synthesis had an enormous influence on the growth of German philosophy and, eventually, on the philosophy of science. The debate was transformed by the growth of modern logic in the late nineteenth and early twentieth centuries, and by the work of philosophers such as Gottlob Frege, Bertrand Russell, and Ludwig Wittgenstein. The twentieth-century debate about the nature of scientific knowledge — in the work of Karl Popper, Thomas Kuhn, and others — is in many ways a continuation of the rationalist-empiricist debate in new terms. The broader context is treated in the page on Philosophy of Science and in the broader Scientific Revolution overview.
A Practical Synthesis
In practice, the rationalist and empiricist traditions were complementary rather than opposed. The successful natural philosophers of the Scientific Revolution — Galileo, Descartes, Newton, Huygens, Boyle, Leibniz — used both reason and experience, both deduction and induction, both mathematics and observation. The debate about which is more fundamental is, in the end, a matter of philosophical interpretation rather than a practical question. The method that worked was a method that combined the two. The story of that combination is told in the article on What Is the Scientific Method? and in the page on Philosophy of Science.