In the youth of the world, men dance and sing and imitate natural objects, observing in these actions, as in all others, a certain rhythm or order. And, although all men observe a similar, they observe not the same order, in the motions of the dance, in the melody of the song, in the combinations of language, in the series of their imitations of natural objects. For there is a certain order or rhythm belonging to each of these classes of mimetic representation, from which the hearer and the spectator receive an intenser and purer pleasure than from any other: the sense of an approximation to this order has been called taste by modern writers.... Language, color, form, and religious and civil habits of action are all the instruments and materials of poetry; and they may be called poetry by that figure of speech which considers the effect as a synonym of the cause. But poetry in a more restricted sense expresses those arrangements of language, and especially metrical language, which are created by that imperial faculty whose throne is curtained within the invisible nature of man. And this springs from the nature itself of language, which is a more direct representation of the actions and passions of our internal being, and is susceptible of more various and delicate combinations than color, form, or motion, and is more plastic and obedient to the control of that faculty of which it is the creation.... Sounds as well as thoughts have relation both between each other and towards that which they represent, and a perception of the order of those relations has always been found connected with a perception of the order of the relations of thoughts. Hence the language of poets has ever affected a certain uniform and harmonious recurrence of sound, without which it were not poetry, and which is scarcely less indispensable to the communication of its influence than the words themselves, without reference to that peculiar order.... An observation of the regular mode of the recurrence of harmony in the language of poetical minds, together with its relation to music, produced metre, or a certain system of traditional forms of harmony and language. Yet it is by no means essential that a poet should accommodate his language to this traditional form, so that the harmony, which is its spirit, be observed. (SHELLEY: _A Defence of Poetry_.) 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."