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.