The demands made by the form of poetry upon its substance are similar to those made by music upon the words in a song, only less stringent. The content must be emotional and significant; it cannot be trite and cold. The meaning of words would permit the poet to bring before the mind all possible objects, events, and ideas, but the music of words would be incongruous with most of them. Events narrated must be stirring, thoughts uttered must be emotionally toned, things described must be related to human life and action. Poetry may desert the royal themes of long ago--_arma virumque cano, maenin aeide thea_--and relate the lowly life of common folk, even the sordid life of the poor and miserable, but when doing so throws over it the musical glamour of verse and arouses the heat of sympathy and passion. Although, since it makes use of words, poetry should always have a meaning, it need not have the definiteness of meaning of logical thought; it may suggest rather than explicate; its music is compatible with vagueness. But vagueness is not obscurity; the poet should always make us feel that we understand him; he should not seek to mystify us, or keep us guessing at his meaning. Yet, since the poet operates with words and not with mere sounds, great subtlety and precision of thought are possible in poetry, although not argument and dialectic. Poetry may express the results of reflection, so far as they are of high emotional value, but cannot well reproduce its processes; the steps of analysis and inference are too cold and hard for the muse to climb. On the other hand, poetry does not permit of the development and iteration of pure feeling which we find in music; for poetic rhythms and melodies lack the variety and fluency of the musical. Yet poetry is capable, where music is not, of expressing brief, quick outbursts of feeling; for a few words, by referring to the causes and conditions of feeling, may adequately express what music needs time and many tones to convey. Poetry wins beauty by concentration, whereas music gains by expansion. There is also a similar relation between prose and poetry in this respect; the severity of the form imposes upon poetry a simplicity which contrasts with the breadth and complexity of prose. As Schopenhauer remarked, every good poem is short; long poems always contain stretches either of unmusical verse or unpoetic music. Yet, in comparison with prose, the tempo of music is slow; we have to linger in the medium in order that its rhythmic and tonal beauties may impress us, and this slowness of movement is imparted to the thought; even narrative and dramatic poetry suffer retardation; for which reason the poetic form must be abandoned if great rapidity of expression is sought.