This change in the language of men is partly the cause and partly the effect of a corresponding change in the nature of their intellectual operations, of a change by which science gains and poetry loses. Generalisation is necessary to the advancement of knowledge; but particularly is indispensable to the creations of the imagination. In proportion as men know more and think more, they look less at individuals and more at classes. They therefore make better theories and worse poems. They give us vague phrases instead of images, and personified qualities instead of men. They may be better able to analyse human nature than their predecessors. But analysis is not the business of the poet. His office is to portray, not to dissect. He may believe in a moral sense, like Shaftesbury; he may refer all human actions to self-interest, like Helvetius; or he may never think about the matter at all. His creed on such subjects will no more influence his poetry, properly so called, than the notions which a painter may have conceived respecting the lacrymal glands, or the circulation of the blood, will affect the tears of his Niobe, or the blushes of his Aurora. If Shakespeare had written a book on the motives of human actions, it is by no means certain that it would have been a good one. It is extremely improbable that it would have contained half so much able reasoning on the subject as is to be found in the Fable of the Bees. But could Mandeville have created an Iago? Well as he knew how to resolve characters into their elements, would he have been able to combine those elements in such a manner as to make up a man, a real, living, individual man? Perhaps no person can be a poet, or can even enjoy poetry, without a certain unsoundness of mind, if anything which gives so much pleasure ought to be called unsoundness. By poetry we mean not all writing in verse, nor even all good writing in verse. Our definition excludes many metrical compositions which, on other grounds, deserve the highest praise. By poetry we mean the art of employing words in such a manner as to produce an illusion on the imagination, the art of doing by means of words what the painter does by means of colours. Thus the greatest of poets has described it, in lines universally admired for the vigour and felicity of their diction, and still more valuable on account of the just notion which they convey of the art in which he excelled. "As imagination bodies forth The forms of things unknown, the poet's pen Turns them to shapes, and gives to airy nothing A local habitation and a name."