Similarity of meaning is “the most important lexical relation” in the WordNet model (Miller and Fellbaum 1991: 202), and, in philosophy, Quine (1961: 22) has identified synonymy (along with analyticity) as “the primary business of the theory of meaning.” This relation raises issues of similarity that remain relevant in the following chapters. In addition, it presents an interesting challenge for the metalexical approach: Can synonymy, a relation among words with similar meanings, be defined in terms of contrast?

Section 4.1 starts the chapter with a description of synonymy in the metalexical approach. This involves a restatement of Relation by Contrast in order to apply it specifically to word sets that differ in form but are otherwise similar enough to be synonyms. Here the ideas presented in chapter 2 are further developed with reference to the notions of ‘similarity’ and ‘difference.’ The next two sections cover the key concepts in synonym studies, so that they can be reinterpreted from the metalexical perspective. Subtypes and degrees of synonymy are examined in 4.2. While some approaches attempt to reduce the number of relations that can count as synonymy, the approach taken here is inclusive. Section 4.3 reviews alleged (logical or traditional) properties of synonymy, such as transitivity and interchangeability, and some evidence against them.