John Locke’s Essay presents a detailed, systematic philosophy of mind and thought. The Essay wrestles with fundamental questions about how we think and perceive, and it even touches on how we express ourselves through language, logic, and religious practices. In the introduction, entitled The Epistle to the Reader, Locke describes how he became involved in his current mode of philosophical thinking. He relates an anecdote about a conversation with friends that made him realize that men often suffer in their pursuit of knowledge because they fail to determine the limits of their understanding.
In Book I, Locke lays out the three goals of his philosophical project: to discover where our ideas come from, to ascertain what it means to have these ideas and what an idea essentially is, and to examine issues of faith and opinion to determine how we should proceed logically when our knowledge is limited. Locke attacks previous schools of philosophy, such as those of Plato and Descartes, that maintain a belief in a priori, or innate, knowledge. He begins by opposing the idea that we are all born knowing certain fundamental principles, such as “whatever is, is.” The usual justification for this belief in innate principles is that certain principles exist to which all human beings universally assent. Locke contends that, on the contrary, no principle is actually accepted by every human being. Furthermore, if universal agreement did exist about something, this agreement might have come about in a way other than through innate knowledge. Locke offers another argument against innate knowledge, asserting that human beings cannot have ideas in their minds of which they are not aware, so that people cannot be said to possess even the most basic principles until they are taught them or think them through for themselves. Still another argument is that because human beings differ greatly in their moral ideas, moral knowledge must not be innate. Finally, Locke confronts the theory of innate ideas (along the lines of the Platonic Theory of Forms) and argues that ideas often cited as innate are so complex and confusing that much schooling and thought are required to grasp their meaning. Against the claim that God is an innate idea, Locke counters that God is not a universally accepted idea and that his existence cannot therefore be innate human knowledge.
Having eliminated the possibility of innate knowledge, Locke in Book II seeks to demonstrate where knowledge comes from. He proposes that knowledge is built up from ideas, either simple or complex. Simple ideas combine in various ways to form complex ideas. Therefore, the most basic units of knowledge are simple ideas, which come exclusively through experience. There are two types of experience that allow a simple idea to form in the human mind: sensation, or when the mind experiences the world outside the body through the five senses, and reflection, or when the mind turns inward, recognizing ideas about its own functions, such as thinking, willing, believing, and doubting.
Locke divides simple ideas into four categories: (1) ideas we get from a single sense, such as sight or taste; (2) ideas created from more than one sense, such as shape and size; (3) ideas emerging from reflection; and (4) ideas arising from a combination of sensation and reflection, such as unity, existence, pleasure, pain, and substance. Locke goes on to explain the difference between primary and secondary qualities. Ideas of primary qualities—such as texture, number, size, shape, and motion—resemble their causes. Ideas of secondary qualities do not resemble their causes, as is the case with color, sound, taste, and odor. In other words, primary qualities cannot be separated from the matter, whereas secondary qualities are only the power of an object to produce the idea of that quality in our minds.
Locke devotes much of book II to exploring various things that our minds are capable of, including making judgments about our own perceptions to refine our ideas, remembering ideas, discerning between ideas, comparing ideas to one another, composing a complex idea from two or more simple ideas, enlarging a simple idea into a complex idea by repetition, and abstracting certain simple ideas from an already complex ideas. Locke also discusses complex ideas, breaking them down into four basic types: (1) modes, which are ideas that do not exist in and of themselves, such as qualities, numbers, and other abstract concepts; (2) substances, either self-subsisting things (such as a particular man or a sheep) or collections of such things (an army of men or a flock of sheep); (3) relations, such as father, bigger, and morally good; and (4) abstract generals, such as “man” or “sheep” in general. Complex ideas are created through three methods: combination, comparison, and abstraction.
In book III, Locke discusses abstract general ideas. Everything that exists in the world is a particular “thing.” General ideas occur when we group similar particular ideas and take away, or abstract, the differences until we are left only with the similarities. We then use these similarities to create a general term, such as “tree,” which is also a general idea. We form abstract general ideas for three reasons: it would be too hard to remember a different word for every particular thing that exists, having a different word for everything that exists would obstruct communication, and the goal of science is to generalize and categorize everything.
Locke argues against the notion of essences, a concept that had been widely accepted since at least Plato’s time. Plato argued that we can only recognize individuals as members of a species because we are aware of the essence of that species—for example, we recognize a particular tree as a tree because we understand what a tree is in its essence. Locke argues that essences don’t actually exist as ideal entities but are instead nothing more than the abstract, general ideas that we form about the things we observe, things that actually exist in the world. Human beings decide which differences and similarities they will use to separate and classify particular things into categories—they choose how to define categories rather than discovering the essence of a given species.
Despite having just criticized the traditional concept of essences, Locke decides to adopt the term into his own philosophy and proceeds to distinguish between real essences and nominal essences. Nominal essences are the specific collections of observable properties from which we create an abstract general idea. For example, we observe similarities between many different individual dogs and from these observations form our idea of what a dog is. Real essences are the invisible structures and arrangements of corpuscles or atoms that allow for those observable properties to be observable in the first place. For example, to return to the case of dogs, if we could fully understand the biological structures and processes that make a dog a dog, whether those would include DNA or other things as well, then we would understand the real essence of dogs. Unlike the nominal essence, the real essence has a basis in reality.
Locke moves on to discuss language, pointing out natural weaknesses and common abuses of language. The most significant problem with words is that they do not immediately and obviously mean the same thing to all people. This problem has four main causes: (1) a word may imply a very complex idea, (2) the ideas that words stand for may have no constant standard anywhere in nature to judge them against, (3) the standard that ideas refer to may not be easily known, and (4) the meaning of a word and the real nature of the thing referred to by the word may not be exactly the same. Locke also identifies six common abuses: (1) people often use words without really knowing what these words mean, (2) people use words inconsistently, (3) people purposefully make terms obscure by using old words for new and unusual uses or by introducing new terms without defining them, (4) people mistakenly believe that words refer to things rather than ideas, (5) people try to use words incorrectly to change their meaning, and (6) people assume that others know what they are saying when they are not really being clear. Locke suggests four remedies to counteract the natural shortcomings and the abuses of language: (1) never use a word without having a clear idea of what it means; (2) try to recognize the same meaning for words as others do so that we can communicate with a common vocabulary; (3) if there is the slightest chance that the meaning of your words will be unclear, define your terms; and (4) always use words consistently.
In book IV, Locke addresses the nature of knowledge itself, asking what knowledge is and in what areas we can hope to attain it. For Locke, knowledge is what the mind is able to perceive through reasoning out the connection, or lack of connection, between any two or more of our ideas. Because knowledge only has to do with relations between ideas, which are in the mind, the knowledge we are capable of is not actually knowledge of the world itself. Locke identifies four sorts of agreement and disagreement that reason can perceive to produce knowledge: (1) identity (blue is blue) and diversity (blue is not yellow), (2) relation (two triangles with equal bases located between the same two parallel lines are equal triangles), (3) coexistence (iron is always susceptible to magnets), and (4) realization that existence belongs to the ideas themselves and is not in the mind (the idea of God and of the self). Locke distinguishes between three grades or degrees of knowledge: intuition, when we immediately perceive an agreement or disagreement the moment the ideas are understood; demonstration, which requires some sort of proof; and sensitive knowledge, which is about the existence of an external world, roughly resembling the world as we perceive it.
Locke argues that we can never really develop a system of knowledge in natural philosophy. The best that we can do is to observe certain qualities in the world that tend to occur together on a regular basis. The kind of connection he demands is the sort that we find between properties occurring together regularly in geometrical figures. Although he doesn’t seem to think we will ever be able to know more about the true nature of things, Locke is hopeful that we can understand existence, and the properties of things that exist in the world, much more thoroughly.
Locke outlines three strategies for dealing with the problem of skepticism, or doubt about whether the world exists outside of our minds. This problem arises naturally from Locke’s theory of knowledge. If we only have access to the ideas in our minds, which only exist in our minds, how do we know there is a real world outside of our minds? Locke’s first strategy is to refuse to take the skeptic seriously. Can anyone really doubt, he asks, that there is an external world out there? His second strategy is to say that it doesn’t matter whether we doubt the existence of an outside world or not. All that matters is that we know enough to enable us to get around in the world. His third line of attack involves seven marks of our experience that can best be explained by the existence of an external world: (1) there is a certain realness and strength of clarity to perception of an immediate object that memories or products of the imagination do not have, (2) we cannot get these ideas without the sense organ appropriate to them, (3) we are able to receive ideas of this sort only in certain situations so it cannot be the organs themselves that are responsible for producing the ideas, (4) we receive ideas passively, (5) some ideas are accompanied by pleasure or pain but the memories of those ideas are not, (6) our senses often bear witness to the truth of each other’s reports, and (7) two different people can share the same experience.
Locke argues that almost all of science, with the exception of mathematics and morality, and most of our everyday experience is subject to opinion or judgment. We base our judgments on the similarity between propositions to our own experience and to the experiences we have heard described by others. Locke examines the relation between reason and faith. He defines reason as being the faculty we use to obtain judgment and knowledge. Faith is the acceptance of revelation and has its own truths, which reason cannot discover. Reason, however, must always be used to determine which revelations truly are revelations from God and which are the constructions of man. Finally, Locke divides all of human understanding into three sciences: natural philosophy, or the study of things to gain knowledge; ethics, or the study of how it is best to act; and logic, or the study of words and signs.
Locke effectively shifted the focus of seventeenth-century philosophy from metaphysics to the more basic problems of epistemology, or how people are able to acquire knowledge and understanding. Locke rigorously addresses many different aspects of human understanding and of the mind’s functions. His most striking innovation in this regard is his rejection of the theory that human beings are born possessing innate knowledge, which philosophers such as Plato and Descartes had sought to prove.
Locke replaces the theory of innate knowledge with his own signature concept, the tabula rasa, or blank slate. Locke tries to demonstrate that we are born with no knowledge whatsoever—we are all blank slates at birth—and that we can only know that things exist if we first experience them.
Locke builds a strong case against the existence of innate knowledge, but the model of knowledge he proposes in its place is not without flaws. By emphasizing the necessity of experience as a prerequisite for knowledge, Locke downplays the role of the mind and neglects to adequately address how knowledge exists and is retained in the mind—in other words, how we remember knowledge and what happens to our knowledge when we aren’t thinking about it and it is temporarily out of our consciousness. While Locke is thorough in his discussion of what objects of experience can be known, he leaves us with little idea of how the mind works to translate experiences into knowledge and to combine certain experiences with other bits of knowledge to categorize and interpret future information.
Locke presents “simple” ideas as a basic unit of human understanding, claiming that we can break all of our experiences down into these simple, fundamental parts that cannot be broken down any further. For example, the idea of a plain wooden chair can be broken down into simpler units that are received by our minds through one sense, through multiple senses, through reflection, or through a combination of sensation and reflection. “Chair” is thus perceived and understood by us in several ways: as brown, as hard, as according to its function (to be sat upon), and as a certain shape that is unique to the object “chair.” These simple ideas allow us to understand what “chair” is and to recognize it when we come in contact with it.
Locke’s theory of primary and secondary qualities is based on the Corpuscular Hypothesis of Robert Boyle, Locke’s friend and contemporary. According to the Corpuscular Hypothesis, which Locke considered the best scientific picture of the world in his day, all matter is composed of tiny particles, or corpuscles, which are too small to see individually and which are colorless, tasteless, soundless, and odorless. The arrangement of these invisible particles of matter gives an object of perception both its primary and secondary qualities. An object’s primary qualities include its size, shape, and movement. They are primary in the sense that these qualities exist regardless of whether anyone perceives them. Secondary qualities include color, odor, and taste, and they are secondary in the sense that they may be perceived by observers of the object, but they are not inherent in the object. For example, a rose’s shape and the way it grows are primary because they exist regardless of whether they are observed, but the rose’s redness only exists for an observer under the right conditions of lighting and if the observer’s eyesight is functioning normally. Locke suggests that because we can explain everything using the existence only of corpuscles and primary qualities, we have no reason to think that secondary qualities have any real basis in the world.
According to Locke, every idea is an object of some action of perception and thinking. An idea is an immediate object of our thoughts, something we perceive and to which we are actively paying attention. We also perceive some things without ever thinking about them, and these things do not continue to exist in our minds because we have no reason to think about them or remember them. The latter are nonimmediate objects. When we perceive an object’s secondary qualities, we are actually perceiving something that does not exist outside of our minds. In each of these cases, Locke would maintain that the act of perception always has an internal object—the thing that is perceived exists in our mind. Moreover, the object of perception sometimes exists only in our minds. One of the more confusing aspects of Locke’s discussion is the fact that perception and thinking are sometimes, but not always, the same action. To add to the confusion, Locke claims in Book II that an action of perception may have a nonimmediate object, not that it must have one. This makes it difficult to pin down a rule for what perception is and isn’t, and how perception works.
We may find Locke’s discussion of essence, or substance, confusing because Locke himself doesn’t seem convinced of its existence. Locke may have chosen to retain this concept for several possible reasons. First, he seems to think that the idea of essence is necessary to make sense of our language. Second, the concept of essence solves the problem of persistence through change: that is, if a tree is just a bundle of ideas such as “tall,” “green,” “leaves,” and so on, what happens when a tree is short and leafless? Does this new collection of qualities change the essence from “tree” to something new? In Locke’s view, the essence persists through any change, remaining the same despite changes in the object’s properties. A third reason Locke seem to be compelled to accept the notion of essence is to explain what unifies ideas that occur at the same time, making them into a single thing, distinct from any other thing. Essence helps clarify this unity, though Locke is not very specific about how this works. For Locke, essence is what qualities are dependent on and exist in.
Locke’s view that our knowledge is much more limited than was previously supposed was shared by other seventeenth- and eighteenth-century thinkers such as Descartes and Hume—even though Locke differs sharply from Descartes about why that knowledge is limited. For Locke, however, the fact that our knowledge is limited is a philosophical rather than practical matter. Locke points out that the very fact that we do not take such skeptical doubts about the existence of the external world seriously is a sign of how overwhelmingly probable we feel the existence of the world to be. The overwhelming clarity of the idea of an external world, and the fact that it is confirmed by everybody except madmen, is important to Locke in and of itself. Even so, Locke holds that we can never have real knowledge when it comes to natural science. Rather than encouraging us to stop bothering with science, Locke seems to say instead that we should be aware of its limitations.