Locke, the great empiricist and pioneer in associationist psychology, was born in Somerset, England, studied medicine and science at Oxford, and later became adviser and physician to the Earl of Shaftesbury. His first works were Essay on a Law of Nature and controversial writings on political and civil questions, for which he was exiled to Amsterdam for several years. During a discussion with friends in 1671 on a “remote” subject, he became convinced that “difficulties that arose on every side” made it “necessary to examine our own abilities and see what objects our understandings were or were not fitted to deal with.” Throughout the following eighteen years he devoted himself to this problem, publishing his conclusions in his most celebrated work, Essay Concerning Human Understanding (1690).Locke returned to England shortly before publishing the Essay, taking a position as Commissioner of Appeals in excise cases. In the years that followed he published two other influential works, Two Treatises on Government and A Letter Concerning Toleration, both of which had a profound effect on the American Constitution. His other major works include Some Thoughts Concerning Education (1693), An Appeal for a Rational Interpretation of the Gospels, and Reasonableness of Christianity (1695).In his Essay Concerning Human Understanding, Locke followed the lead of Hobbes in opposing the doctrine of innate ideas which Descartes had supported. He also developed the theory of association which Hobbes had outlined, and formulated the empirical approach to philosophy and psychology. The key concept of the work is the term “idea,” or “object of thinking,” which he described as a basic unit of mind or item of knowledge. According to Locke, our conscious, ongoing behavior can be analyzed at any moment into these units, such as man, dog, hardness, and sweetness. In his view the mind is a tabula rasa (blank sheet of paper) at birth, and every one of these ideas—and therefore all our knowledge—derives from sensory experience. This doctrine of empiricism was in direct opposition to the rationalism of Descartes, who held that the mind arrives on the scene alreadyequipped with certain “clear and distinct” ideas.Locke recognized that ideas were not isolated elements, but combine into totalities of varying degrees of complexity and abstractness. To explain the way they are “held together,” he proposed the doctrine of association of ideas. According to this theory, associations are established between ideas primarily by “custom”—that is, by habitually perceiving them together—just as certain configurations of colors, shapes, odors, etc. mean “dog,” and a set of abstract concepts combine to form the idea of “autocracy.” A basic principle of the associationist philosophy is that ideas may be either simple and un- analyzable (the russet color of a setter, for example), or complex and analyz- able into simpler components (for example, government). Locke was not altogether clear about how the process of compounding or “mental chemistry” takes place, but he made a distinction between modes (for instance, different kinds of triangles), substances (dog, house), and relations, which arise when we compare one idea with another (larger, or brighter). These processes were more precisely described by later associationists.Two other aspects of Locke’s doctrines had a particularly strong influence on the development of psychology. First, he traced ideas to two sources, sensation and reflection. In sensation, external bodies impinge on the sense organs, which send messages to the mind, producing perceptions. In reflection, the mind gains knowledge of its own operations through an “inner sense.” In this process (similar to introspection), we get ideas about ideas and the manner of their occurrence.This concept was a forerunner of act psychology, which focuses on the operations of the mind itself.The other important contribution was Locke’s distinction between primary and secondary qualities. Primary qualities are perceived singly by the senses (shape, extent) and are believed to stem directly from objects in the external world. Locke viewed them as presentations of properties inherent in the objects themselves—the shape we see corresponds to the actual shape of the object. Secondary qualities, on the other hand, do not exist in the objects in the form in which they are perceived, but are due to the power of the object to produce the ideas in mind. Examples are colors, sounds, • tastes, and smells, all of which are generated by the object but do not resemble it. As Boring (1950) points out, “This distinction between primary and secondary qualities means that the mind is not always a mirror of the external world, but gains much of its knowledge about reality indirectly.” Though Locke himself did not describe this process in specific terms, his views are believed to foreshadow both the doctrine of specific energy and the Gestalt theory of isomorphism. See ASSOCIATIONISM, SPECIFIC ENERGIES, ACT PSYCHOLOGY, GESTALT PSYCHOLOGY.