Book III closes with some odds and ends on the topic of language. In chapter vii, Locke examines the origin of our connective words, such as "is" and "and." Connective words, unlike all other words, do not refer to ideas but to actions of the mind. Locke ends Book III by looking at the natural weaknesses of language, the common abuses of language, and the remedies for linguistic weakness and abuse.
With regard to linguistic imperfection, Locke identifies one major weakness, which he then breaks down into four primary causes. The major imperfection of words is that sometimes they do not excite in the hearer the same idea that the speaker is trying to convey. To use the same example as before, assume I am giving a speech on the evils of "sexual harassment," and that by "sexual harassment," I only mean overt sexual assault. If I fail to define the term, the idea that I conjure up in the minds of the audience might well be very different from the idea I am actually speaking about. To them, "sexual harassment" can include anything from overt sexual assault to an ill-taken compliment. Locke claims that there are four instances in which words are particularly liable to result in this sort of miscommunication: if they are very complex; if the ideas they stand for have no settled standard anywhere in nature to judge them against; if the standard that they refer to is not easily known; or if the meaning of the word and the real essence of the thing are not exactly the same. Names of mixed modes (such as in the example above) are most prone to imperfections arising out of the first two reasons, and the names of substances are prone to imperfections arising out of the latter two. In addition to this natural imperfection of words, Locke also identified six common abuses. First, people often use words without having any distinct idea of what these words are meant to signify (either because these words never had any clear and distinct ideas attached to them, or else because they are being used sloppily). Second, people use words inconsistently. Third, people purposely make terms obscure, either by applying old words to new and unusual references, or else by introducing new and ambiguous terms without defining them. Fourth, people mistakenly believe that words refer to things rather than ideas. Fifth, people try to use words to signify things they cannot or do not signify. Lastly, people assume that others know what they mean by their words when really it is not at all clear what they mean. Locke suggests four remedies to counteract the natural imperfections and the abuses of words. First, one should never use a word without having a clear idea of what it means. Second, one should try to assign words the same meaning that others assign them. Third, if there is any chance of ambiguity, one should define one's terms. Finally, one should always use words consistently.
A difficulty arises from Locke's account of connective words. If these words stand for the actions of the mind, then we must apprehend these actions of the mind in some way. Presumably, if we apprehend these actions, it is by way of ideas, since, according to Locke, ideas always intervene in any act of perception. However, if we have ideas for these actions of the mind, there seems to be no reason why our connectives do not signify these ideas. All other words, after all, signify ideas. Presumably, then, Locke does not believe we have ideas corresponding to the connective actions of the mind. Unfortunately, this leads to an even greater inconsistency because it requires that we perceive the actions of the mind without intervening ideas, something that Locke has expressly stated is impossible. Locke has a sticky choice here: either he can explain why connective words, unlike all other words, refer to actions rather than the ideas of those actions, or else, if he chooses to claim that in this case there are no ideas to which the words can refer, he must admit that, in at least one case, perception can occur without intervening ideas. He would then have to explain how such an idea-less perceptive act could even take place on his view.