Few, if any, books have had as great an impact on the history of thought on the nature of human consciousness as John Locke's Essay Concerning Human Understanding (1690). Hardly a book on the subject is written in England from the time of its publication through the Romantic period which does not respond in some way to Locke's text. The text itself is a comprehensive and detailed analysis of the mechanisms of human thought. An analysis which Locke believed had the potential to shed new light on social and religious thought. In the course of the book, Locke himself attempts to use his model to explain many philosophical dilemmas, such as the relationship between the material world, subjectivity and the divine. The books most sustained influence on British thought was, however, not a result of his philosophizing about these relationships, but rather of his initial impetus to categorize and describe the relationship between various types of human thought. It is the nature of of the model of cognition that Locke developes which lead John Stuart Mill, for example, to dub him the "unquestioned founder of the analytic philosophy of mind." Many later British thinkers would disagree with Locke, but few would be able to refrain from constructing their own cognitive models as justifications for their social, aesthetic, or religious philosophies in the face of the weight of the Influence of Locke's Essay.
The foundation of Locke's cognitive model is his division of human thinking into a series of interrelated but distinct processes --each with its own parameters and functions. According to Locke, all thinking can be understood first to fall into one of the two general categories of SENSATION or REFLECTION --Sensation describing the way in which "our senses, conversant bout particular sensible objects, do convey into the mind several distinct perceptions of things, according to those various ways wherein those objects do affect them" (122), and Reflection being "the perception of the operations of our own mind within us, as it is employed about the ideas it has got" (123). Believing that the mind is, at birth, an "empty cabinet" (48) or a sheet of "white paper, void of all characters, without any ideas" (121), he claims that these two modes of thinking, Sensations and Reflection, are "the only originals from whence all our ideas take their beginnings" (124). Reflection can not, however, occur except as there are thoughts present to reflect upon. Thus, for Locke, all thinking begins with Sensation --"The perception is the first operation of all our intellectual faculties, and the inlet of all our knowledge" (191).
Locke describes the process by which the senses furnish the mind with its first thoughts as a function of mediation. Walking a line between the skeptical and materialist dualist extremes, Locke claims that "the ideas of primary (material) qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves, but the ideas produced in us by these secondary qualities have no resemblance of them at all" (173). The material world does, according to Locke exist, but we only have access through the mediation of certain secondary material qualities (such as motion, refraction, heat, etc.) and therefore have an 'idea' of its materiality but no real knowledge of it. It is these "ideas" which are, according to Locke, the fundamental building blocks of all human thought. Locke is, thus, strictly speaking neither a pure skeptic nor materialist, but advocates a postion which mediates between the two.
Having made the initial division of thinking into the two "operations" of Sensation and Reflection, and having described the way in which Sensation furnishes the mind with its primary material, Locke notes that while Sensation is at its root a passive process (i.e., a sound wave hits the ear drum and produces an automatic response), Reflection can be active or passive and can actually intervene into the operation of Sensation (183, 186, 298). Having pointed this out, Locke goes on to examine and identify several sub-classes of thought which exist under the category of Reflection. This discussion takes the form, first, of a categorization of the types of ideas, the objects with which the reflection busies itself, which are present in the mind. According to Locke, all ideas fall into one of the two general categories of Simple or Complex--simple ideas being those ideas which are "not distinguishable into different ideas" (145), such as hot, cold, white, etc., and Complex ideas being those which are produced by the understanding "repeat[ing], compar[ing], and unit[ing]" (145) simple ideas. He further divides Complex ideas into the sub-categories of Modes, Substances, and Relations (215), defining Modes as what we today would think of as qualities, as power, identity, etc., Substances as things, such as the idea of a Man, and Relations in terms of mathematical relational properties such as squared, triangular, etc.
Locke also defines the many ways in which the mind goes about producing and manipulating these ideas. Having already distinguished between Reflection and Sensation as two broad categories of thought, Locke defines several sub-categories of thought which fall under the category of Reflection, each with its own unique attribute. According to Locke, all Reflective thought falls into one of the following sub-categories: Memory (193) --the ability to recall an absent idea back into consciousness-- Retention (193) --the ability to hold a thought in the consciousness-- Discerning (202) --the ability to recognize the differences between things-- Comparing (204), the ability to recognize the similarities between things--Composition (205) --the ability to construct new ideas from the building blocks of other ideas-- and Abstraction (207) --the ability to discern abstract relational principles, such as mathematical proofs, which lie behind other ideas and therefore create an idea of the general. According to Locke, all human thought is explained by these divisions.
Another aspect of the human psyche with which Locke deals is that of the Will. Locke accounts for the existence of the human will by asserting that humans are basically hardwired to experience the sensations of pain and pleasure and that all action is the result of a drawing towards the one or moving away from the other. He writes, "Pain has the same efficacy and use to set us on work that pleasure has, we being as ready to employ our faculties to avoid that, as to pursue this" (161). Later in the work, however, the creator mysteriously drops out of the picture and man becomes capable of choosing for himself what is pleasurable or painful (363), and only the hardwired mechanism which drives man towards pleasure and a way from pain remains a product of the Creator. Unfortunately, Locke never adequately describes where exactly the impetus to will such a change originates.
Although Hobbes is the first major figure to attempt to build a philosophy which coherently and logically lead from the material, through the individual subject and society, to the spiritual, Locke's treatise is the first provide a detailed model of the mechanisms of human thought in this context. Literally volumes have been written on Locke's work, so rather than discuss the internal aspects of his Essay I will focus instead on its importance to other thinkers of the next 150 years. As Alexander Fraser has pointed out, "The art of education, political thought, theology and philosophy, especially in Britain, France, and America, long bore the stamp of the Essay, or of reaction against it, to an extent that is not explained by the comprehensiveness of Locke's thought, or by the force of his genius" (xi). Locke's thinking is impressive, but not sufficiently so to account for his pervasiveness as an influence. There is, however, something unique about Locke's methodology which is structurally reflected in his philosophical progeny and which could account for his popularity. Hobbes was the first to suggest that the laws of politics and religion could be explained by the same logic that was used to discover mathematical truths; but Locke was the first to apply this logic so completely to the study of human subjectivity. Locke frequently conflates questions of subjectivity with discussion of mathematical proof. For example, in his discussion of the question of the existence of innate moral principles Locke writes, "But this is no derogation to their truth and certainty; no more than it is to the truth or certainty of the tree angles of a triangle being equal to two right ones because it is not so evident as the 'whole is bigger than a part; nor so apt to be assented to at first hearing. It may suffice that these moral rules are capable of demonstration" (65). In fact, Locke's entire method is one of importing the rules of geometric 'demonstration' to the realm of social psychology. Locke's entire treatise is in reality nothing short of a geometric 'proof' in which a series of theorems are proved using a series of deductions based upon logical axioms or theorems which have previously in the text been demonstrated to be true. It is for this reason, more than for the details of his actual demonstration, that we think of Locke as the father of empirical psychology. Anyone after him who wished to enter the arena of mainstream philosophical thought would have to deal with the rhetorical force of this methodology either by meeting Locke on his terms or by openly refusing to participate. Empericism could be denied, but not ignored.