The modern secular tradition initiated by the Renaissance was advanced by the Scientific Revolution and the Enlightenment.  This chapter surveys the major thinkers and ideas of these ages and discusses the principles of the modern outlook to which they contributed.

Combining Ptolemeic astronomy, Aristotelian philosophy, and Christian theology, medieval thought posited a hierarchical, geocentric universe.  Renaissance humanists and artists began to challenge this view by reviving classical learning and by promoting accurate mathematical representation of the physical world.  Inspired by ancient scientific texts, the seventeenth-century scientific thinkers gradually replaced the old Ptolemeic-Aristotelian model of the universe with a Pythagorean-Platonic model.  Copernicus took the first steps in this effort when he challenged the cumbersome Ptolemeic system with his Platonic heliocentric theory.  Proposing the uniformity of nature, Galileo grounded Copernicus' theory in careful observations of planets, moons, and stars.  Galileo also challenged Aristotelian physics through pioneering experiments with motion.  In an effort to separate science from faith, Galileo attacked uncritical acceptance of ancient authority, an attack that prompted the church to denounce his ideas and silence him.  Further supporting Copernicus' theory, Kepler's laws fused Pythagorean and Platonic principles into a mathematically harmonious account of planetary motion.  Answering the questions Kepler could not, Newton developed a physics that explained mathematically all the phenomena of motion within the heliocentric system.  Newton also pioneered the modern science of optics and promoted experimental technique, all while arguing that God was the architect of the mechanical universe.

As this new model of the universe evolved, a new approach to scientific inquiry emerged.  Denying the traditional philosophical authorities, Bacon advocated the inductive approach, arguing that conclusions should be based on observable facts alone.  Descartes promoted the deductive approach, proclaiming the mind's ability to formulate incontrovertible first principles from which further knowledge may be derived.  These complementary approaches to rational investigation provided crucial intellectual tools to modern experimental and theoretical science.

The Scientific Revolution erected the modern conception of a homogenous universe, the structure of which could be represented in terms of mathematical relationships and chemical compositions.  Although the pioneers of this view saw no conflict between it and Christianity, that view prompted spiritual anxiety in later centuries, anguish poignantly articulated by Pascal.   Still, the Scientific Revolution and the critical spirit it fostered weakened traditional Christianity and all the concepts of authority that attended it, laying the groundwork for the Enlightenment.

What the Scientific Revolution did for the physical world, the Enlightenment strove to do for society and politics.  Cultivating Cartesian skepticism, the philosophes questioned all received ideas, working thereby to liberate humanity from tyranny and superstition.  Many of the basic terms and concepts of this movement came from the work of Hobbes and Locke.  From Hobbes' secular political theory, the philosophes took the rejection of divine monarchy, while rejecting its pessimism and approval of absolutism.  From Locke, the philosophes borrowed political ideas including natural rights and constitutional government, and the epistemological notion that knowledge comes not from innate ideas, but accumulated experience.

On this foundation, the philosophes built a systematic critique of Western thought and belief.  Denouncing traditional Christianity as brutal and superstitious, the philosophes subjected the Bible to critical scrutiny and rejected the clerical establishment.  Most embraced rational deism, promoting morality over ritual and freedom of conscience over coerced observance.  In the face of censorship, philosophes including Montesqieu, Voltaire, and Rousseau advanced modern political ideas such as governmental checks and balances, the rule of law, and government by general will.  Rejecting mercantilism, Smith proposed an enlightened economics in which governments allow the invisible hand of the marketplace to work unimpeded.  Assuming the essential goodness of human nature, Rousseau developed an educational philosophy that treated children as children, and many Enlightenment thinkers—including Beccaria, Paine, and Franklin—denounced torture, war, and the slave trade.  Although most philosophes considered women inferior to men, their ideas contained the possibility of women's equality, a possibility Wolstonecraft hoped to realize through her critique of women's subordination.  Finally, Enlightenment optimism and veneration of science prompted the idea of continuous human progress articulated most fully by Condorcet.

The modern outlook owes its core principles to the Enlightenment thinkers.  Borrowing the insights and methods of the Scientific Revolution, the philosophes offered a rational, secular interpretation of society that advocated tolerance, freedom, equality, and rule of law.  These ideals came to serve as the theoretical foundation of modern liberal government and market economics.