Sentence:  ...such objects belonged to the domain of the comic poet, and of the lighter kinds of poetry. For the more serious kinds, for <WSD>"pragmatic</WSD>" poetry, to use an excellent expression of Polybius, they were more difficult and severe in the range of subjects which they permitted. 
Meaning of the word marked with <WSD> tags:  Philosophical; dealing with causes, reasons, and effects, rather than with details and circumstances; said of literature.