In practice, we make short cuts by resorting to consideration of meaning and see what forms of similar meanings are also formally similar. On the whole, nouns are names of persons and things, verbs are words expressing events and actions, and so on. Such loose definitions will of course not stand rigorous formal scrutiny, and exceptions are very easy to find. But through the general trend of meanings some formal features may be found to serve as more rigorous definitions.
For example, since most names of persons and things have the formal feature of being modifiable by a D-M compound, that is, a determinative plus a measure, such as 一种 i- joong ‘a kind (of)’, 十对shyr-duey ‘ten pairs (of)’ , we can turn around and define a noun as whatever can be modified by a D-M compound, and then the class of nouns so defined will include not only most names of persons and things but whatever will satisfy the definition, for example, 一种习惯i-joong shyi- guann ‘a kind of habit’, which will make shyiguann a noun, though it denotes neither a person nor an event or action. The meaning or set of meanings for the majority of cases of a given grammatical category is called the class meaning, or grammatical meaning of that category. Once a form class or any other grammatical category is defined formally, then its class meaning (which has often served to suggest the formal definition) will be useful only as a convenient reminder and no longer used as a real criterion.
 Chao Y R. A grammar of spoken Chinese[M]. Univ of California Press, 1965.