Of Berkeley's seven published works, his Essay Towards a New Theory of Vision (1709), Treatise Concerning the Principles of Human Knowledge (1710), and Dialogues Between Hylas and Philonous (1713), contain his most significant work on the structure of human cognition. A student at Trinity College and direct predecessor of Locke, Berkeley's methodology is decidedly Lockean and empirical, although he frames most of his work as a direct refutation of many of Locke's principles. In his Essay Towards a New Theory of Vision, Berkeley provides a detailed analysis of how the two senses of Sight and Touch furnish the mind with Ideas, and briefly discusses the relationship between Ideas and their correspondent reality and the nature of Ideas as the furniture of the mind, directly attacking Locke's conception of Abstract ideas. His Treatise Concerning the Principles of Human Knowledge takes up where his Essay left off and expands his discussion of the impossibility of Abstract Ideas and of the relationship between Ideas and the material world, in which he defends his position against accusations of Skepticism. The Dialogues, the last of Berkeley's serious philosophical works, continue in the line of defense begun in the Treatise. As the title implies, they are written as a dialogue between two thinkers, one of whom supports Berkeley's position and the other who launches a series of attacks against it which reflect the criticisms which Berkeley himself was subject too. By in large, while Berkeley's thought is complex and detailed, and his rhetorical style pleasing, his work shows little conceptual development over time. Rather, he advances a few important philosophical insights early in his thought and spends the remainder of his philosophical career defending and buttressing them.
Berkeley's model begins with the mechanisms of human vision. Refuting earlier theories of vision which suggested that distance and magnitude were optically judged via the mental calculation of changes in the angle of intersection of the lines of sight from each eye along the 'optic axes' (Barrow, Molyneux), Berkeley claims that, in fact, while these distances my be explained by geometry, they are not experienced as such. Rather, he speculates, as an object moves closer or farther from the eyes, the eyes move in order to maintain a focus on the object and the individual 'feels' this movement and through experience begins to associate the relative position of the eye in its socket with a particular conception of distance. There is an important conceptual difference between this model and those against which Berkeley was arguing. In order for those earlier models to function, a person had to have both an innate conception of the idea distance and an innate ability to translate the results of geometric calculations into this conception. Following in the tradition of Locke, Berkeley believed that the mind came into the world a blank slate and that all knowledge was gained through experience and the senses. This belief rendered earlier models of vision non-functional, because the mind could not be thought of as innately having the capability to perform the mathematical calculations which it called for. Berkeley's model, on the other hand, was perfectly capable of explaining the development of a conception of distance. He argued that such a concept evolved simply from the arbitrary but reliable association of things being a certain distance away at the same time that a person felt her/his eyes resting in a certain position. It was, according to Berkeley, through experience and sensation, and not through calculation, that humans developed first the concept of distance and then the ability to predict it.
Experience is a key term in Berkeley's philosophy, for it is through experience of one sort or another that he postulates that we obtain all of our knowledge. First, each individual sense brings to the mind a certain and particular set of ideas dependent upon the nature of the sense. Sight, for example, brings light and color, hearing -sound, and smell -odor, touch -feeling. The different senses are, he claims separate and distinct and have no innate connection to each other. It is only by virtue of the fact that we often simultaneously experience the ideas presented to the various senses upon a certain occasion that we begin to associate these different types of stimuli -color, light, smell, sound- with the same external object. He writes, "It is nevertheless certain, the ideas intromitted by each sense are widely different, and distinct from each other; but having been observed constantly to go together, they are spoken of as one and the same thing" (33). Thus, according to Berkeley, whenever we speak of an Idea of a particular object as having several attributes each belonging to different senses (the ball was round, blue and smooth) we are really not talking about one complete idea, but rather of several unique and independent but associated ideas. (One sees in this formulation an immature version of later associationist thought.). In fact, it is these ideas, which have been made present in the mind by the various senses, which Berkeley claims are actually the only reality to which we have access. "The visible figure of any part of the body hath no necessary conexion with the tangible figure thereof"(61). As such, a person can never really have any experience of a single "thing" which contains all the attributes of light, color, feel, odor and sound simultaneously. Rather, we have access to "units" of thought which are collections of various ideas that, as a result of experience and need, we group together and consider as coherent wholes. "The unit constantly relates to the particular draughts the mind makes of its ideas, to which it affixes names, and wherein it includes more or less as best suits to its own ends and purposes. Whatever therefore the mind considers as one, that is a unit. Every combination of ideas is considered as one thing by the mind, and in token thereof is marked by one name. Now, this naming and combining together of ideas is perfectly arbitrary, and done by the mind in such sort, as experience shows it to be most convenient" (63).
Berkeley's conception of Units, of our access to reality via a series of mentally constructed independent ideas with no natural connection to each other, leads him in his later works to theorize that all reality is actually nothing more than the reality of the world of ideas which exists in the individual mind. In his Essay Towards a New Theory of Vision, for example, he writes, "All the choir of heaven and furniture of the earth, in a word all those bodies which compose the mighty frame of the world, have not any substance without a mind, that their being (esse) is to be perceived or known; that consequently so long as they are not actually perceived by me, or do not exist in my mind or that of any other created spirit, they must either have no existence at all, or else subsist in the mind of some eternal spirit: it being perfectly unintelligible and involving all the absurdity of abstraction, to attribute to any single part of them an existence independent of a spirit" (180). This postulate presents certain problems for Berkeley's earlier conception of the sensuous foundations of all human knowledge. In order to remedy this, Berkeley simply re-defines the nature of the reality with which he claims the senses interact. Believing that "there is no corporeal or material substance: it remains therefore that the cause of ideas is an incorporeal active substance or spirit" (187). Berkeley names this spirit, or group thereof, the "Laws of Nature". "Now the set of rules or established methods, wherein the mind we depend on excites in us the ideas of sense, are called the laws of nature: and these we learn by experience, which teaches us that such and such ideas are attended with such and such other ideas, in the ordinary course of things" (189). Locke's conception of the division of all thought into the two categories of Sensation and Reflection is problematized by this claim, since sensation has moved out of the realm of the experience of the material and into the realm of the cognitive. Berkeley does not, however, claim that sensation and reflection are the same thing. Rather, he claims that there is a difference between the two categories of thought, but that it is a difference of degree rather than of kind. "The ideas of sense are allowed to have more reality in them, that is , to be more (1) strong, (2) orderly, and (3) coherent than the creatures of the mind: but this is no argument that they exist without the mind" (190). The mind is, ultimately, the only reality for Berkeley. [It is important to note that despite how we might categorize this postulate, Berkeley himself overtly denies that his model is skeptical on the grounds that since he believes that skepticism can only exist if, in fact, there exists a material reality from which the mental reality can be severed. Since his model postulates no such reality, it can not, according to him, be skeptical. (see his Treatise, 204-214 and the first of the Dialogues).]
There are obvious contradictions and incongruities in Berkeley's model. Most significant among these is the fact that the materiality of the human body in which the model is initially grounded is completely effaced in its final formulation. Berkeley's entire model rests upon his initial analysis of vision in which abstract knowledge is rooted in the experience of the physical movements of the eye, but if all sensation is mental, then existence of such an experience is brought into question. As Berkeley's analysis progresses, the body, which is where he begins his proof, drops completely out of the picture. Paradoxically, the validity of the model rests upon an assumption that the model ultimately disproves -that of the existence of matter.
Such inconsistencies are significant, but not as significant as the important ways in which Berkeley's thought enters into dialogue with other thinkers from the period. Berkeley's work presents itself as a direct attack on many of the tenets of mainstream British psychological thought since Locke; however, both at a structural and a behavioral level, by which I mean simply the level at which the model can be used to explain and/or predict human behavior, Berkeley suggests little that is different from his contemporaries. It is, rather, at the epistemological level that Berkeley inserts himself into the empirical debate, and the few and subtle differences between Berkeley's model of cognition and those of other empirical thinkers have a significant impact on questions of language, subjectivity and 'reality'. Berkeley begins his inquiry by noting simply that "Philosophy being nothing else but the study of wisdom and truth, it may with reason be expected, that those who have spent the most time in it should enjoy a greater calm and serenity of mind" but that, quite to the contrary, "no sooner do we depart from sense and instinct to follow the light of superior principle ...but a thousand scruples spring up on our mind, concerning those things which before we seemed fully to comprehend" and yet "we see the illiterate bulk of mankind, that walk the high road of plain, common sense, and are governed by the dictates of nature, for the most part easy and undisturbed" (163). From this observation, he deduces not that "right deductions from true principles should ever end in consequences which cannot be maintained" (164), but that the initial principles upon which philosophers of the human condition had until that point based their theories must be flawed, and sets for himself the task of identifying these faulty principles: "to try to discover what those principles are, which have introduced all that doubtfulness and uncertainty, those absurdities and contradictions into the several sects of philosophy" (164).
In the Second Dialogue Between Hylas and Philonous, Berkeley provides a skeletal outline of the prevalent model of cognition against which he sets his own model. He writes:
"It is supposed the soul makes her residence in some part of the brain, from which the nerves take their rise, and are thence extended to all parts of the body: and that outward objects, by the different impressions they make on the organs of sense, communicate certain vibrative motions to the nerves; and these being filled with spirits, propagate them to the brain or seat of the soul, which according to the various impressions or traces thereby made in the brain, is variously affected with ideas" (276).
What is important in this refutation of Locke is not its validity or non-validity, but Berkeley's method of refutation. Berkeley does not question Locke on the grounds of logic, but rather on the ground of experience. He presents less a new examination of the mechanisms of cognition and more a treatment of how Locke's models stands up to a personal, experientially based, metacognitive test. Berkeley's work thus presents itself as an early attempt to resurrect the importance of the experiential subject in the face of a trajectory of empirical discourse which tended to make more generalized and abstract claims about the nature of subjectivity -one which does not speak from outside of the empiricist paradigm but rather seeks to insert experience into that paradigm as a valid data set.