Poetry in its matter and form is natural imagery or feeling, combined with passion and fancy. In its mode of conveyance, it combines the ordinary use of language with musical expression. There is a question of long standing in what the essence of poetry consists, or what it is that determines why one set of ideas should be expressed in prose, another in verse. Milton has told us his idea of poetry in a single line: Thoughts that voluntary move Harmonious numbers. As there are certain sounds that excite certain movements, and the song and dance go together, so there are, no doubt, certain thoughts that lead to certain tones of voice, or modulations of sound, and change "the words of Mercury into the songs of Apollo". There is a striking instance of this adaptation of the movement of sound and rhythm to the subject, in Spenser's description of the Satyrs accompanying Una to the cave of Sylvanus: So from the ground she fearless doth arise, And walketh forth without suspect of crime. They, all as glad as birds of joyous prime, Thence lead her forth, about her dancing round, Shouting and singing all a shepherd's rhyme; And with green branches strewing all the ground, Do worship her as queen with olive garland crown'd. And all the way their merry pipes they sound, That all the woods and doubled echoes ring; And with their horned feet do wear the ground, Leaping like wanton kids in pleasant spring; So towards old Sylvanus they her bring, Who with the noise awaked, cometh out.