Although they both lived and worked in the late 17th century, Sir Isaac Newton and John Locke (1632–1704) were the true fathers of the Enlightenment. Newton was the last of the scientific geniuses of the age, and his great Philosophiae Naturalis Principia Mathematica (1687; Mathematical Principles of Natural Philosophy) was the culmination of the movement that had begun with Copernicus and Galileo—the first scientific synthesis based on the application of mathematics to nature in every detail. The basic idea of the authority and autonomy of reason, which dominated all philosophizing in the 18th century, was, at bottom, the consequence of Newton’s work.
Copernicus, Kepler, Bacon, Galileo, and Descartes—scientists and methodologists of science—performed like people urgently attempting to persuade nature to reveal its secrets. Newton’s comprehensive mechanistic system made it seem as if at last nature had done so. It is impossible to exaggerate the enormous enthusiasm that this assumption kindled in all of the major thinkers of the late 17th and 18th centuries, from Locke to Kant. The new enthusiasm for reason that they all instinctively shared was based not upon the mere advocacy of philosophers such as Descartes and Leibniz but upon their conviction that, in the spectacular achievement of Newton, reason had succeeded in conquering the natural world.
Classical British empiricism
Two major philosophical problems remained: to provide an account of the origins of reason and to shift its application from the physical universe to human nature. Locke’s Essay Concerning Human Understanding (1690) was devoted to the first, and Hume’s Treatise of Human Nature (1739–40), “being an attempt to apply the method of experimental reasoning to moral subjects,” was devoted to the second.
These two basic tasks represented a new direction for philosophy since the late Renaissance. The Renaissance preoccupation with the natural world had constituted a certain “realistic” bias. Hobbes and Spinoza had each produced a metaphysics. They had been interested in the real constitution of the physical world. Moreover, the Renaissance enthusiasm for mathematics had resulted in a profound interest in rational principles, necessary propositions, and innate ideas. As attention was turned from the realities of nature to the structure of the mind that knows it so successfully, philosophers of the Enlightenment focused on the sensory and experiential components of knowledge rather than on the merely mathematical. Thus, whereas the philosophy of the late Renaissance had been metaphysical and rationalistic, that of the Enlightenment was epistemological and empiricist. The school of British empiricism—John Locke, George Berkeley (1685–1753), and David Hume—dominated the perspective of Enlightenment philosophy until the time of Kant.
Reason in Locke and Berkeley
Locke’s An Essay Concerning Human Understanding marked a decisively new direction for modern philosophizing because it proposed what amounts to a new criterion of truth. Locke’s aim in his essay—“to inquire into the origin, certainty, and extent of human knowledge”—involved three tasks:
- To discover the origin of human ideas.
- To determine their certainty and evidential value.
- To examine the claims of all knowledge that is less than certain.
What was crucial for Locke, however, was that the second task is dependent upon the first. Following the general Renaissance custom, Locke defined an idea as a mental entity: “whatever is the object of the understanding when a man thinks.” But whereas for Descartes and the entire rationalist school the certainty of ideas had been a function of their self-evidence—i.e., of their clarity and distinctness—for Locke their validity depended expressly on the mode and manner of their origin. Thus, an intrinsic criterion of truth and validity was replaced with a genetic one.
Locke’s exhaustive survey of mental contents is useful, if elaborate. Although he distinguished between ideas of sensation and ideas of reflection, the thrust of his efforts and those of his empiricist followers was to reduce the latter to the former, to minimize the originative power of the mind in favour of its passive receptivity to the sensory impressions received from without. Locke’s classification of ideas into “simple” and “complex” was an attempt to distinguish mental contents that are derived directly from one or more of the senses (such as blueness or solidity, which come from a single sense such as sight or touch, and figure, space, extension, rest, and motion, which are the product of several senses combined) from complicated and compounded ideas of universals (such as triangle and gratitude), substances, and relations (such as identity, diversity, and cause and effect).
Locke’s Essay was a dogged attempt to produce the total world of human conceptual experience from a set of elementary sensory building blocks, moving always from sensation toward thought and from the simple to the complex. The basic outcome of his epistemology was therefore:
- That the ultimate source of human ideas is sense experience.
- That all mental operations are a combining and compounding of simple sensory materials into complex conceptual entities.
Locke’s theory of knowledge was based upon a kind of sensory atomism, in which the mind is an agent of discovery rather than of creation, and ideas are “like” the objects they represent, which in turn are the sources of the sensations the mind receives. Locke’s theory also made the important distinction between “primary qualities” (such as solidity, figure, extension, motion, and rest), which are real properties of physical objects, and “secondary qualities” (such as colour, taste, and smell), which are merely the effects of such real properties on the mind.
It was precisely this dualism of primary and secondary qualities that Locke’s successor, George Berkeley, sought to overcome. Although Berkeley was a bishop in the Anglican church who professed a desire to combat atheistic materialism, his importance for the theory of knowledge lies rather in the way in which he demonstrated that, in the end, primary qualities are reducible to secondary qualities. His empiricism led to a denial of abstract ideas because he believed that general notions are simply fictions of the mind. Science, he argued, can easily dispense with the concept of matter: nature is simply that which human beings perceive through their sense faculties. This means that sense experiences themselves can be considered “objects for the mind.” A physical object, therefore, is simply a recurrent group of sense qualities. With this important reduction of substance to quality, Berkeley became the father of the epistemological position known as phenomenalism, which has remained an important influence in British philosophy to the present day.
Basic science of human nature in Hume
The third, and in many ways the most important, of the British empiricists was the skeptic David Hume. Hume’s philosophical intention was to reap, humanistically, the harvest sowed by Newtonian physics, to apply the method of natural science to human nature. The paradoxical result of this admirable goal, however, was a skeptical crisis even more devastating than that of the early French Renaissance.
Hume followed Locke and Berkeley in approaching the problem of knowledge from a psychological perspective. He too found the origin of knowledge in sense experience. But whereas Locke had found a certain trustworthy order in the compounding power of the mind, and Berkeley had found mentality itself expressive of a certain spiritual power, Hume’s relentless analysis discovered as much contingency in mind as in the external world. All uniformity in perceptual experience, he held, comes from “an associating quality of the mind.” The “association of ideas” is a fact, but the relations of resemblance, contiguity, and cause and effect that it produces have no intrinsic validity because they are merely the product of “mental habit.” Thus, the causal principle upon which all knowledge rests represents no necessary connections between things but is simply the result of their constant conjunction in human minds. Moreover, the mind itself, far from being an independent power, is simply “a bundle of perceptions” without unity or cohesive quality. Hume’s denial of a necessary order of nature on the one hand and of a substantial or unified self on the other precipitated a philosophical crisis from which Enlightenment philosophy was not to be rescued until the work of Kant.