Whereas in most languages nouns and verbs are distinct lexical categories, there are also languages like Samoan, in which such a distinction does not seem to serve any descriptive purpose in the grammar. This contribution is an attempt to discover what distinguishes languages in which nouns and verbs are separate word classes from languages without a rigid noun/verb distinction. I will argue that transitivity plays an essential role in the parts-of-speech systems of languages across the globe in that a language can only have distinct classes of nouns and verbs if a subgroup of the basic lexical items in a language are semantically coded as designating a transitive relationship. There is a difference, however, in that the presence of a set of transitive items in the basic lexicon is a necessary and sufficient condition for a language to have a major, distinct class of verbs, but only a necessary condition for a language before it can have a major, distinct class of nouns. Ultimately I will argue that a language can only have distinct classes of verbs, nouns, and adjectives if the basic meaning of lexical items somehow encodes the prototypical properties of temporal and spatial entities (events and things). The prototypical event is an activity that involves an agent and a patient; the prototypical thing is a concrete object. Thus, a language can only have major, distinct classes of verbs, nouns and adjectives if the lexicon contains (a) items that designate a dynamic relationship between an agent and a patient, and (b) items that designate a property that is specified as having a boundary in the spatial dimension.