In this era dedicated to human progress, the advancement of the natural sciences is regarded as the main exemplification of, and fuel for, such progress. Isaac Newton's epochal accomplishment in his Principia Mathematica (1687), which, very briefly described, consists in the comprehension of a diversity of physical phenomena in particular the motions of heavenly bodies, together with the motions of sublunary bodies in few relatively simple, universally applicable, mathematical laws, was a great stimulus to the intellectual activity of the eighteenth century and served as a model and inspiration for the researches of a number of Enlightenment thinkers. Newton's system strongly encourages the Enlightenment conception of nature as an orderly domain governed by strict mathematical-dynamical laws and the conception of ourselves as capable of knowing those laws and thus plumbing the secrets of nature through the exercise of our unaided faculties. The conception of nature, and of how we know it, changes significantly with the rise of modern science. It belongs centrally to the agenda of Enlightenment philosophy to contribute to the new knowledge of nature, and to provide a metaphysical framework within which to place and interpret this new knowledge.

Ren Descartes' rationalist system of philosophy is foundational for the Enlightenment in this regard. Descartes (15961650) undertakes to establish the sciences upon a secure metaphysical foundation. The famous method of doubt Descartes employs for this purpose exemplifies (in part through exaggerating) an attitude characteristic of the Enlightenment. According to Descartes, the investigator in foundational philosophical research ought to doubt all propositions that can be doubted. The investigator determines whether a proposition is dubitable by attempting to construct a possible scenario under which it is false. In the domain of fundamental scientific (philosophical) research, no other authority but one's own conviction is to be trusted, and not one's own conviction either, until it is subjected to rigorous skeptical questioning. With his method, Descartes casts doubt upon the senses as authoritative source of knowledge. He finds that God and the immaterial soul are both better known, on the basis of innate ideas, than objects of the senses. Through his famous doctrine of the dualism of mind and body, that mind and body are two distinct substances, each with its own essence, the material world (allegedly) known through the senses becomes denominated as an external world, insofar as it is external to the ideas with which one immediately communes in one's consciousness. Descartes' investigation thus establishes one of the central epistemological problems, not only of the Enlightenment, but also of modernity: the problem of objectivity in our empirical knowledge. If our evidence for the truth of propositions about extra-mental material reality is always restricted to mental content, content immediately before the mind, how can we ever be certain that the extra-mental reality is not other than we represent it as being? The solution Descartes puts forward to this problem depends on our having prior and certain knowledge of God. In fact, Descartes argues that all human knowledge (not only knowledge of the material world through the senses) depends on metaphysical knowledge of God.

However dubious Descartes' grounding of all scientific knowledge in metaphysical knowledge of God, his system contributes significantly to the advance of natural science in the period. He attacks the long-standing assumptions of the scholastic-aristotelians whose intellectual dominance stood in the way of the development of the new science; he developed a conception of matter that enabled mechanical explanation of physical phenomena; and he developed some of the fundamental mathematical resources in particular, a way to employ algebraic equations to solve geometrical problems that enabled the physical domain to be explained with precise, simple mathematical formulae. Furthermore, his grounding of physics, and all knowledge, in a relatively simple and elegant rationalist metaphysics provides a model of a rigorous and complete secular system of knowledge. Though it is typical of the Enlightenment thinkers of the eighteenth century (for example Voltaire in his Letters on the English Nation, 1734) to embrace Newton's physical system in preference to Descartes', Newton's system itself depends on Descartes' earlier work, a dependence of which Newton himself was aware.

Cartesian philosophy is also foundational for the Enlightenment through igniting various controversies in the latter decades of the seventeenth century that provide the context of intellectual tumult out of which the Enlightenment springs. Among these controversies are the following: Are mind and body two distinct sorts of substances, as Descartes argues, and if so, what is the nature of each, and how are they related to each other, both in the human being (which presumably has both a mind and a body) and in a unified world system? If matter is inert (as Descartes claims), what can be the source of motion and the nature of causality in the physical world? And of course the various epistemological problems: the problem of objectivity, the role of God in securing our knowledge, the doctrine of innate ideas, et cetera.

Baruch Spinoza's systematic rationalist metaphysics, which he develops in his Ethics (1677) in part in response to problems in the Cartesian system, is also an important basis for Enlightenment thought. Spinoza develops, in contrast to Cartesian dualism, an ontological monism according to which there is not only one kind of substance, but one substance, God or nature, with two attributes, corresponding to mind and body. Spinoza's denial, on the basis of strict philosophical reasoning, of the existence of a transcendent supreme being, his identification of God with nature, gives strong impetus to the strands of atheism and naturalism that thread through Enlightenment philosophy. Spinoza's rationalist principles also lead him to assert a strict determinism and to deny any role to final causes or teleology in explanation. (See Israel 2001.)

The rationalist metaphysics of Leibniz (16461716) is also foundational for the Enlightenment, particularly the German Enlightenment (die Aufklrung), which is founded to a great extent on the Leibnizean rationalist system of Christian Wolff (16791754). Leibniz articulates, and places at the head of metaphysics, the great rationalist principle, the principle of sufficient reason, which states that everything that exists has a sufficient reason for its existence. This principle exemplifies the faith, so important for the Enlightenment, that the universe is fully intelligible to us through the exercise of our natural powers of reason. The problem arises, in the face of skeptical questioning, of how this principle itself can be known or grounded. Wolff attempts to derive it from the logical principle of non-contradiction (in his First Philosophy or Ontology, 1730). Criticism of this alleged derivation gives rise to the general question of how formal principles of logic can possibly serve to ground substantive knowledge of reality. Whereas Leibniz exerts his influence through scattered writings on various topics, some of which elaborate plans for a systematic metaphysics which are never executed by Leibniz himself, Wolff exerts his influence on the German Enlightenment through his development of a rationalist system of knowledge in which he attempts to demonstrate all the propositions of science from first principles, known a priori.

Wolff's rationalist metaphysics is characteristic of the Enlightenment by virtue of the pretensions of human reason within it, not by reason's success in establishing its claims. Much the same could be said of the great rationalist philosophers of the seventeenth century. Through their articulation of the ideal of scientia, of a complete science of reality, composed of propositions derived demonstratively from a priori first principles, these philosophers exert great influence on the Enlightenment. But they fail, rather spectacularly, to realize this ideal. To the contrary, what they bequeath to the eighteenth century is metaphysics, in the words of Kant, as a battlefield of endless controversies. However, the controversies themselves regarding the nature of God, mind, matter, substance, cause, et cetera, and the relations of each of these to the others provide tremendous fuel to Enlightenment thought.

Despite the confidence in and enthusiasm for human reason in the Enlightenment it is sometimes called the Age of Reason the rise of empiricism, both in the practice of science and in the theory of knowledge, is characteristic of the period. The enthusiasm for reason in the Enlightenment is not for the faculty of reason as an independent source of knowledge (at least not primarily), which is actually put on the defensive in the period, but rather for the human cognitive faculties generally; the Age of Reason contrasts with an age of religious faith, not with an age of sense experience. Of course, as outlined above, the great seventeenth century rationalist metaphysical systems of Descartes, Spinoza and Leibniz exert significant influence on philosophy in the Enlightenment; moreover, the eighteenth-century Enlightenment has a rationalist strain, perhaps best exemplified by the system of Christian Wolff. Still, that the Encyclopedia of Diderot and D'Alembert is dedicated to three empiricists, Francis Bacon, John Locke and Isaac Newton, indicates the general ascendency of empiricism in the period.

If the founder of the rationalist strain of the Enlightenment is Descartes, then the founder of the empiricist strain is Francis Bacon (15611626). Though Bacon's work belongs to the Renaissance, the revolution he undertook to effect in the sciences inspires and influences Enlightenment thinkers. The Enlightenment, as the age in which experimental natural science matures and comes into its own, admires Bacon as the father of experimental philosophy. Bacon's revolution (enacted in, among other works, The New Organon, 1620) involves conceiving the new science as (1) founded on empirical observation and experimentation; (2) arrived at through the method of induction; and (3) as ultimately aiming at, and as confirmed by, enhanced practical capacities (hence the Baconian motto, knowledge is power).

Though each of these elements of Bacon's revolution is significant for natural science in the Enlightenment, the point about method deserves special emphasis. Granted that Newton's work stands as the great exemplar of the accomplishments of natural science for the eighteenth century, the most salient contrast between Newton's work and that of the great rationalist systems lies in their methods. Whereas the great rationalist philosophers of the seventeenth century conceive of scientific knowledge of nature as consisting in a system in which statements expressing the observable phenomena of nature are deduced from first principles, known a priori, Newton's method begins with the observed phenomena of nature and reduces its multiplicity to unity by induction, that is, by finding mathematical laws or principles from which the observed phenomena can be derived or explained. The contrast between the great success of Newton's bottom-up procedure and the seemingly endless and fruitless conflicts among philosophers regarding the meaning and validity of first principles of reason naturally favors the rise of the Newtonian (or Baconian) method of acquiring knowledge of nature in the eighteenth century.

Excerpt from:

Enlightenment (Stanford Encyclopedia of Philosophy)