Language is not merely a convenience — it is the medium through which human social life is possible. But its power to bind people together depends on the assumption that the words different people use correspond to the same ideas. Locke is deeply suspicious of this assumption. Words, he argues, signify ideas in the minds of those who use them, and there is no guarantee that different speakers associate the same ideas with the same words. This gap between words and ideas is the source of most philosophical disputes that seem irresolvable.
Locke's account of abstract ideas is among the most discussed in the Essay. We experience only particular things — this horse, that triangle. Yet we use general terms — "horse", "triangle" — that are supposed to apply to many particulars. How? Locke's answer is abstraction: the mind takes the idea of a particular thing, strips away the features that make it particular, and retains a general idea that applies to all things of that kind. The abstract idea of "triangle" is not any specific triangle but a representation of the features common to all triangles.
Berkeley later attacked this account, arguing that there are no genuinely abstract ideas — that we cannot form a mental image of "triangle in general" with no determinate angle-sizes or proportions. Locke's defenders replied that abstract ideas are not images but general representations. The dispute marks one of the central controversies of early modern philosophy about the nature of conceptual thought.
The practical consequence of Locke's analysis is a catalogue of linguistic abuses: using words without any clear idea attached to them; using the same word with different ideas on different occasions; using words whose ideas are obscure or confused; disputing over words when the real question is about things. Much of what passes for philosophical controversy is, Locke suggests, controversy about words — a confusion that careful attention to the ideas behind the words would dissolve. Book III is as much a manual of intellectual hygiene as a philosophical analysis of language.
Book III of the Essay Concerning Human Understanding is often treated as a digression from the main epistemological argument, but Locke regarded it as essential. His analysis of language and abstract ideas was directly contested by Berkeley in the Treatise Concerning the Principles of Human Knowledge.