179 Croatian Journal of Philosophy Vol. XIV, No. 41, 2014 A Semantic Theory of Word Classes PETER GÄRDENFORS Cognitive Science, Lund University Sydney Technical University Within linguistics a word class is defined in grammatical terms as a set of words that exhibit the same syntactic properties. In this paper the aim is to argue that the meanings of different word classes can be given a cognitive grounding. It is shown that with the aid of conceptual spaces, a geometric analysis can be provided for the major word classes. A uni- versal single-domain thesis is proposed, saying that words in all content word classes, except for nouns, refer to a single domain. Keywords: Conceptual spaces, cognitive semantics, word class- es, single domain, convexity, noun, adjective, verb, preposition, adverb. 1. Cognitively grounded semantics 1 What is it that you know when you know a language? Certainly you know many words of the language—its lexicon; and you know how to put the words together in an appropriate way—its syntax. More impor- tantly, you know the meaning of the words and what they mean when put together into sentences. In other words, you know the semantics of the language. If you do not master the meaning of the words you are using, there is no point in knowing the syntax. Therefore, as regards communication, semantic knowledge is more fundamental than syn- tactic. (I am not saying the syntax does not contribute to the meaning of a sentence, only that without knowledge of the meanings of the basic words there is no need for syntax.) In Gärdenfors (2014), I connect the semantics of various forms of communication to other cognitive processes, in particular concept for- mation, perception, attention, and memory. As Jackendoff (1983: 3) puts it: “[T]o study semantics of natural language is to study cognitive 1 This article is, to a large extent, a summary of material in Part 2 of Gärdenfors (2014).