The roots of the Enlightenment are generally traced to the Renaissance, the Protestant Reformation and the Scientific Revolution. The fourteenth-century Italian Renaissance was sparked by the rediscovery of classical Greek writings and featured a return to secular concerns, in sharp contrast to the theological focus of medieval thought. The Reformation, which began with Martin Luther’s posting of his ninety-five theses in 1517, challenged the traditional authority of the Roman Catholic Church, claiming that individuals could acquire religious truth through their own interpretation of the scriptures. The Scientific Revolution, which is usually dated from the sixteenth century as well, was an endeavor to ascertain truth through a scientific, that is, a rational and empirical, method.

The Reformation and the Scientific Revolution shared a commitment to the pursuit of truth by individuals who exercised reason rather than passively accepting traditional authority. To a considerable extent, then, both of these movements were concerned with epistemology, or the theory of how knowledge is obtained. They emphasized skepticism, individualism, and reason. In common with the Renaissance, they were primarily secular in orientation.

In the seventeenth and eighteenth centuries, these currents of thought produced two distinct schools of philosophy, which combined to form the Enlightenment. The first school, known as rationalism, began with the work of René Descartes. Descartes adopted a position of radical skepticism, questioning everything and accepting nothing on authority. He then attempted to determine whether there was anything that could be known with certainty and found only one thing: he could know for certain his own doubt. From the fact of his doubt, he deduced his own existence—a deduction sometimes referred to as the cogito, after the famous proposition Cogito, ergo sum (I think, therefore I am). Descartes went on to develop an entire philosophy, including an explanation of the existence of God, by a series of deductions originating with the cogito. Descartes’ model of epistemology was mathematics, a system that begins with certain intuitive truths and then arrives at further truths by a process of deduction. Descartes rejected as unreliable the information provided by one’s senses because he found that appearances often deceive us—things are not as they seem.

In Cartesian philosophy, then, the pursuit of knowledge originates with the individual who refuses to accept any truth that cannot be established by his own exercise of reason. Cartesian rationalism dominated European philosophy in the seventeenth and eighteenth centuries and is represented, for example, in the work of Benedict de Spinoza and Gottfried Leibniz.

By beginning with his own self-consciousness, Descartes separated his mind from the tangible things of the world. He first proved the existence of his own thoughts, then reasoned from that to the existence of his body and the other objects in the world. Thus, built into Descartes’ epistemology is a distinction between one’s individual, subjective consciousness and the objective world existing beyond one’s mind.

The other school of philosophy that arose out of the Reformation and the Scientific Revolution was empiricism, whose most important figures include three seventeenth- and eighteenth-century British empiricist philosophers, John Locke, George Berkeley, and David Hume. Locke, too, began with a skeptical posture. He believed, however, that knowledge came not through Cartesian introspection, but rather through the experience of the senses. In Locke’s view, the only things we can know for certain are those that we can learn empirically. Berkeley and Hume each pushed the implications of Locke’s empiricism a step further. Berkeley argued that to exist is to be perceived. Because abstract ideas cannot be sensed, they cannot exist outside the mind. Hume took the position that because we experience only isolated sensations, no principle can be known through induction and no causal link between phenomena can be established with certainty. All we can know is the particulars that we experience. Our notion that these particulars demonstrate a unifying principle or a chain of causation is merely a psychological predisposition, a habitual assumption that the particulars are related, not something we can definitely know.

Despite the disagreement between Descartes and Locke concerning whether intuition or sense impression are the starting points of knowledge, the rationalists and empiricists made certain common assumptions growing out of Descartes’ work. Both Descartes and Locke subordinated metaphysics to epistemology, meaning that they started with a theory of knowledge from which they constructed their account of reality. Both began their inquiry with the individual. Descartes began with individual intuition, whereas Locke began with individual sense impressions. With both, however, the individual was at the center of their philosophy. Finally, in beginning with the individual, both the rationalists and the empiricists assumed a distinction between the subjective mind of the individual and the objective world outside of individual consciousness.

Rationalism and empiricism represent the two strands of Enlightenment epistemology. Enlightenment thinkers believed that we can know only by reason or by experience, the methods used by science. They turned away from tradition, authority, and revelation as a means of knowing the world. The Enlightenment thus represented an attempt to apply the scientific method to all forms of knowledge. The Enlightenment image of the natural world was inspired by Isaac Newton’s laws of physics: the universe is an elaborate machine, like a clock, that operates in a regular fashion according to a natural law established by the Creator. The eighteenth-century Enlightenment has been called the Age of Reason and, at least at the time of the American Revolution, the Cartesian rationalist strand dominated the empirical strand, particularly in political philosophy.

Medieval philosophers had divided in their metaphysics between the realists and the nominalists. The realists believed that general concepts have a real existence and traced their lineage to Plato and his belief in the existence of “forms” that are the universal essences of particular earthly objects. The nominalists believed that only particulars actually exist and that general concepts are merely names that humans apply to groups of particulars. As early as the fourteenth century, William of Ockham had associated nominalism with empiricism in a way that anticipated Berkeley and Hume. Although the work of the British empiricists brought a nominalist strand to Enlightenment philosophy, the dominant metaphysic in the late eighteenth century was realist: the universe is governed by natural laws that reason can perceive.

Liberal political theory, particularly as expounded by its most influential theoretician, John Locke, held that individuals have natural rights to liberty and equality, that individuals through a social contract agree to form governments for the sole purpose of protecting those rights, and that the only legitimate government is based on the consent of the governed. Liberal economic theory asserted that a market economy allowing individuals the freedom to pursue their rational self-interest would produce greater wealth than the heavily regulated economies of mercantilism.

In Locke’s view, the purpose of government was the protection of individual liberty. Locke also wrote about the form that government should take, arguing for a separation of powers among different branches of government so that no one branch would dominate. After studying the British constitutional system, Baron de Montesquieu refined Locke’s separation of powers theory, calling for a division of government into the legislative, executive, and judicial branches.

The fundamental premise of Lockean liberalism was that individuals have rights based on natural law. A belief in natural law was traceable to the Stoics of ancient Greece and had been incorporated into medieval theology, which understood natural law to be law based on the will of God as revealed in scripture and to be binding on all people.

Belief in a natural law that governed human affairs also fit comfortably in the Enlightenment image of the universe as an elaborate machine governed by Newtonian laws of physics. People, like nature, were subject to a natural law. Thus, Thomas Jefferson in the Declaration of Independence could proclaim the existence of “self-evident” truths concerning the equality of men at birth and their natural right to life, liberty, and the pursuit of happiness. This natural law, however, was to be ascertained through the exercise of intuition and reason in the tradition of Descartes rather than to be derived from religious revelation. Natural law had acquired a basis in secular thought.

The epistemological and political theory of the Enlightenment converged in the late eighteenth century to produce the experiment of the American Constitution. Reduced to its essence, eighteenth-century American legal philosophy might be characterized as the use of reason to protect liberty. Through the exercise of reason, men could discern their natural rights, some of which they codified in the Bill of Rights of the U.S. Constitution.

So intense was the belief that these natural rights had genuine existence that many considered their codification in a Bill of Rights superfluous and even dangerous, because it could lead courts to assume that only the codified rights were enforceable. In the years following the Revolution, courts claimed the right to invalidate legislation that was contrary to natural law, even though not prohibited by anything in the Constitution.

Eighteenth-century American legal philosophy held that the common law was similarly based on principles that could be discerned by a court through the exercise of reason. These case law principles included the natural rights of men, and thus the common law posed no threat to liberty. In fact, eighteenth-century Americans regarded the common law as a source of liberty, precisely because it embodied long-established principles, and they challenged the right of British Parliament to modify those principles through legislation, such as the imposition of taxes without the consent of the taxed.

The institutional structure that ensured that reason would protect liberty was the separation of powers. Judges, who in theory were guided by reason rather than political passion, would review the constitutionality of legislation or apply the common law. Judges as rational decision makers would guarantee the preservation of liberty.

Reason would thus protect liberty through the rule of law. Courts were not to decide cases in accordance with the individual preferences of the judge but in accordance with the dictates of law ascertained by reason.

Naturalism was also challenged by the utilitarian philosophy of Jeremy Bentham and John Stuart Mill. Bentham argued that a society should seek the greatest happiness for the greatest number, a philosophy that seemed congenial to majoritarian democracy because it argued for the welfare of the society as a whole rather than for the inalienable rights of individuals. The greatest happiness for the greatest number could be inconsistent with the interests of any particular individual, and thus utilitarian philosophy was deeply corrosive of any naturalist belief in individual rights. Individual rights would be protected, if at all, not because of the worth of the individual, but because to do so would benefit the society as a whole. Utilitarianism suggested that courts should not look backward to determine what preexisting rights were determinative of a dispute but should look forward to the consequences of each possible resolution of the dispute.