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Percy Bysshe Shelley believed that poetry could excite the imagination, which, as the source of compassion and benevolence, could in turn improve society. In his 'A Defence of Poetry', published posthumously by his wife Mary Shelley (neé Wollstonecraft Godwin) in 1840, he argued:
'A man, to be greatly good, must imagine intensely and comprehensively; he must put himself in the place of another and of many others. The pains and pleasures of his species must become his own. The great instrument of moral good is the imagination; and poetry administers to the effect by acting upon the cause. Poetry enlarges the circumference of the imagination by replenishing it with thoughts of ever new delight, which have the power of attracting and assimilating to their own nature all other thoughts, and which form new intervals and interstices whose void forever craves fresh food. Poetry strengthens the faculty which is the organ of the moral nature of man, in the same manner as exercise strengthens a limb.'
Thus poets had an important role to play in society; they were, in his view, the 'unacknowledged legislators of the world':
'[P]oets, or those who imagine and express this indestructible order, are not only the authors of language and of music, of the dance, and architecture, and statuary, and painting: they are the institutors of laws, and the founders of civil society, and the inventors of the arts of life, and the teachers, who draw into a certain propinquity with the beautiful and the true that partial apprehension of the agencies of the invisible world which is called religion [...] It is impossible to read the compositions of the most celebrated writers of the present day without being startled with the electric life which burns within their words. They measure the circumference and sound the depths of human nature with a comprehensive and all-penetrating spirit, and they are themselves perhaps the most sincerely astonished at its manifestations; for it is less their spirit than the spirit of the age. Poets are the hierophants of an unapprehended inspiration; the mirrors of the gigantic shadows which futurity casts upon the present; the words which express what they understand not; the trumpets which sing to battle, and feel not what they inspire; the influence which is moved not, but moves. Poets are the unacknowledged legislators of the world.'