Observation and imagination are the basic principles of all poetry. It is impossible to conceive a poetical work from which one of them is wholly absent. Observation without imagination makes for obviousness; imagination without observation turns into nonsense. What marks the world's greatest poetry is perhaps the presence in almost equal proportion of both these principles. But as a rule we find one of them predominating, and from this one-sided emphasis the poetry of the period derives its character as realistic or idealistic.