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."
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." ... The jerks, the breaks, the inequalities and harshnesses of prose are fatal to the flow of a poetical imagination, as a jolting road or a stumbling horse disturbs the reverie of an absent man. But poetry "makes these odds all even." It is the music of language, answering to the music of the mind, untying, as it were, "the secret soul of harmony." Wherever any object takes such a hold of the mind as to make us dwell upon it, and brood over it, melting the heart in tenderness, or kindling it to a sentiment of enthusiasm; wherever a movement of imagination or passion is impressed on the mind, by which it seeks to prolong and repeat the emotion, to bring all other objects into accord with it, and to give the same movement of harmony, sustained and continuous, or gradually varied, according to the occasion, to the sounds that express it—this is poetry. The musical in sound is the sustained and continuous; the musical in thought is the sustained and continuous also. There is a near connection between music and deep-rooted passion. Mad people sing. As often as articulation passes naturally into intonation, there poetry begins.... It is to supply the inherent defect of harmony in the customary mechanism of language, to make the sound an echo to the sense, when the sense becomes a sort of echo to itself—to mingle the tide of verse, "the golden cadences of poetry," with the tide of feeling, flowing and murmuring as it flows—in short, to take the language of the imagination from off the ground; and enable it to spread its wings where it may indulge its own impulses:
"Sailing with supreme dominion
Through the azure deep of air,"
without being stopped, or fretted, or diverted with the abruptnesses and petty obstacles, and discordant flats and sharps of prose, that poetry was invented. It is to common language what springs are to a carriage, or wings to feet. In ordinary speech we arrive at a certain harmony by the modulations of the voice; in poetry the same thing is done systematically by a regular collocation of syllables. It has been well observed, that every one who declaims warmly, or grows intent upon a subject, rises into a sort of blank verse or measured prose.... An excuse might be made for rime in the same manner. It is but fair that the ear should linger on the sounds that delight it, or avail itself of the same brilliant coincidence and unexpected recurrence of syllables, that have been displayed in the invention and collocation of images. It is allowed that rime assists the memory.... But if the jingle of names assists the memory, may it not also quicken the fancy?
(William Hazlitt: On Poetry in General.)
With regard to the principle of Variety in Uniformity by which verses ought to be modulated, and oneness of impression diversely produced, it has been contended by some that poetry need not be written in verse at all; that prose is as good a medium, provided poetry be conveyed through it, and that to think otherwise is to confound letter with spirit, or form with essence. But the opinion is a prosaical mistake. Fitness and unfitness for song, or metrical excitement, just make all the difference between a poetical and prosaical subject; and the reason why verse is necessary to the form of poetry is that the perfection of poetical spirit demands it—that the circle of its enthusiasm, beauty, and power is incomplete without it. I do not mean that a poet can never show himself a poet in prose; but that being one, his desire and necessity will be to write in verse; and that, if he were unable to do so, he would not and could not deserve his title. Verse to the true poet is no clog. It is idly called a trammel and a difficulty. It is a help. It springs from the same enthusiasm as the rest of his impulses, and is necessary to their satisfaction and effect.... Verse is the final proof to the poet that his mastery over his art is complete. It is the shutting up of his powers in "measureful content"; the answer of form to his spirit; of strength and ease to his guidance.... Verse, in short, is that finishing, and rounding, and "tuneful planeting" of the poet's creations which is produced of necessity by the smooth tendencies of their energy or inward working, and the harmonious dance into which they are attracted round the orb of the beautiful. Poetry, in its complete sympathy with beauty, must of necessity leave no sense of the beautiful, and no power over its forms, unmanifested; and verse flows as inevitably from this condition of its integrity as other laws of proportion do from any other kind of embodiment of beauty....