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.