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.