An Essay Concerning Human Understanding begins with a short epistle to the reader and a general introduction to the work as a whole. Following this introductory material, the Essay is divided into four parts, which are designated as books. Book I has to do with the subject of innate ideas. This topic was especially important for Locke since the belief in innate ideas was fairly common among the scholars of his day. The belief was as old as the dialogues of Plato, in which the doctrine of a world of ideas or universals had been expressed. Plato had taught that ideas are latent in the human mind and need only the stimulation of sense perception to bring them to the level of consciousness. Many of the philosophers of the so-called rationalistic school followed Plato in this respect.
In the era that preceded Locke, Descartes had insisted that the criterion of truth was to see so clearly and distinctly that it could not be doubted. For him the source of all knowledge was to be found in these ideas, which because they were innate, were also true. From them all other truths could be derived by making logical inferences. Locke saw many of the difficulties that follow from this position, and it occurred to him that these could be avoided if it could be shown conclusively that innate ideas do not exist. Any attempt to further the cause of human knowledge must begin by showing the falsity of this position. This is what he attempted to do in Book I.
A more affirmative aspect of this theory of knowledge was set forth in Book II. Having stated his reasons for rejecting the belief in innate ideas, he now goes on to show how it is possible to construct the whole pattern of human knowledge from what has been experienced. Beginning with an account of simple ideas which are derived from the senses, he proceeds to an explanation of the ideas of reflection, perception, space, time, substance, power, and others that are related to these.
Book III has to do with the meanings of words. It includes analysis of general terms, the names of simple ideas, the names of substances, an account of abstract and concrete terms, and a discussion concerning the abuse of words.
Book IV treats the subjects of knowledge and probability. Some information is given about knowledge in general, and this leads to a discussion with reference to the degrees of knowledge and the extent of human knowledge. In addition, it includes a detailed account of such subjects as the reality of knowledge, the nature of truth, the character of judgments, and the respective roles of reason and faith.
Locke's theory of knowledge as a whole may be said to have four dominant characteristics. These are empiricism, dualism, subjectivism, and skepticism. A brief word concerning each of these should be helpful in preparing one to read the entire book.
Locke's empiricism was to a large extent the result of the contrast he had observed between the natural scientists of his day and the work of the moralists and theologians. The conclusions advanced by the scientists were tentative and always subject to revision in the light of new facts. Moralists and theologians were usually of the opinion that their doctrines expressed the final and absolute truth, and no amount of experimentation or observation would cause them to change. The scientists were making remarkable progress and, with all of their differences, were discovering more and more areas of agreement.
No similar progress could be observed in the areas of morals and religion. Indeed, there seemed to be more confusion and disagreements here than in other fields of inquiry. What was the reason for all of this? The answer, as Locke saw it, was to be found in the different methods that had been used.
The scientists did not begin with some innate idea or presupposition from which their knowledge could be derived. Instead, they looked to experience as the sole source of information, and they accepted as true only those conclusions that could be verified by experiment and observation.
The moralists and theologians had used a different method. They began with some authoritative statement. It might be an innate idea, as it was in the philosophy of Descartes, or it could be a divine revelation or something that was so regarded by an ecclesiastical body. Whatever was accepted in this fashion necessarily became the source from which knowledge must be derived. Since this knowledge could be obtained by deductive inference from the initial starting point, it was believed to have a certainty and finality about it that would not be possible on any other basis.
People who believe they have certain or absolute knowledge are likely to be intolerant of those who hold opposite opinions. Intolerance leads to persecution and the suppression of human freedom. In view of these considerations, it seemed clear to Locke that the method employed by the scientists was the only safe one to follow and that this method should be extended to cover all fields of inquiry.
In his acceptance of the empirical method used by the scientists, Locke took over some of their basic presuppositions as well. One of these was the belief in an external world the existence of which is quite independent of what human minds may know about it. Although he remained somewhat skeptical about the nature of that which is external to the mind, he followed the customary procedure among the scientists of referring to it as a material world. On the other hand, knowledge and all that is included in human consciousness were regarded as the world of mind, something that was separate and distinct from the world of matter.
This dualism of mind and matter was comparable to that of a knowing subject and an object which is known. Just how these two worlds, which are so different in their respective characteristics, can interact on one another is something that Locke did not explain, but that an interaction of some kind did take place he never doubted. It had been recognized for some time that the sense qualities of color, sound, taste, and so forth, do not belong to the objects that are sensed but to the mind which perceives the objects. At the same time, it was generally assumed that spatial characteristics and such items as size, weight, and density are present in the objects which constitute the material world.
Locke followed the customary practice of designating the qualities that belong only to the mind as secondary and those that belong to the objects as primary. Recognizing the difficulty that is involved in knowing anything at all about the real nature of that which is external to the mind, he assumed that, whatever its nature might be, it was capable of acting on human minds and causing the sensations that are experienced.
Having accepted the empirical method as the only reliable one for an adequate understanding of the phenomenon of human knowledge, Locke was led by the logic of his position into a kind of subjectivism. This means that one may have genuine knowledge about only the workings of the human mind, and consequently no positive claims can be made about the nature of that which lies outside the sphere of consciousness. This may seem to be a strange position for him to take since the scientists whose methods he was attempting to follow always considered that they were studying the material world and not merely the appearances which it produced in human minds.
Locke's major contribution in this respect consisted in shifting the emphasis from a study of nature to a study of the mind and the processes by means of which knowledge of any kind is obtained. In doing this, he achieved a measure of success, for he was able to give some account of the way in which ideas are formed even though he was unable to present any empirical evidence for assertions concerning the nature of that which is external to the mind.
It is obvious that the logical outcome of Locke's empirical method could be nothing other than skepticism insofar as the real nature of the external world is concerned. While it is true that Locke continued to believe in many of the basic assumptions of the scientists of the seventeenth century, he could provide no evidence from human experience to support their validity. He believed as ardently as any of the scientists that there is a rational order in nature and a cause and effect relationship which holds good for all observed phenomena. But since these beliefs imply more than the facts of experience, we may have faith in their validity but we can have no certain knowledge concerning them. Because the term knowledge had been used in a way that implied certainty, Locke was forced to the conclusion that we can have no genuine knowledge about nature. All that we can have is probable knowledge.
This conclusion he did not think should cause any alarm, nor should it be disturbing to any thoughtful person. Probable knowledge is, in many areas at least, reliable knowledge, and as such it is sufficient for our needs. Since this is true, we ought not to bemoan the fact that our minds are limited. Rather, we should learn to make use of what capabilities we do possess. The only certain knowledge that we have is the kind which is illustrated in the field of mathematics, where the test of truth is the consistency of our ideas with one another. But this type of knowledge does not tell us anything about the world of nature, nor does it give us truths in the areas of morals and religion.