In his "Defence of Poetry" Shelley says that the imagination is the moral instrument. To be greatly good a man must imagine intensely and comprehensively. Poetry serves morality not by what it explicitly teaches, but by its power to awaken and enlarge the mind, to render it "the receptacle of a thousand unapprehended combinations of thought." Since poetry strengthens the imagination, which is the organ of the moral nature of man, "a poet would do ill to embody his own conceptions of right and wrong, which are usually those of his time and place, in his poetical creations which participate in neither." A remarkable book could be made of the best things said in prose by English poets about poetry. Perhaps one book would not hold so much. A narrower yet great and imaginative book could be made of what Shelley said about poetry and what English poets have said about him. Such a book would explain and exhibit the theory of poetry and the art of criticism. The very good edition of Shelley in the Regent Library, (edited by Roger Ingpen) contains some brief "Testimonia" which invite one to the essays from which they are taken, by Browning, Swinburne, Francis Thompson. It is significant that Mr. Ingpen has not quoted from Arnold. If it is the function of poetry to expand the imagination and make the mind aware of a thousand unapprehended combinations of thought, how did it happen that Arnold, a genuine poet, missed Shelley utterly? Arnold was not satisfied with his essay and intended to return to the subject. That he could do a better thing is proved by his essay on Keats, which, after he has done with his droning, schoolmasterly defence of Keats's morals, is eloquent, serene and restrainedly emotional. Shelley phrased many of the revolutionary ideas that were current in his time. Arnold's timid school-bred culture was impervious to any sort of revolutionary idea. Shelley's ideas did not impress him; he thought Shelley a wonderful singer, but a singer without a solid body of thought. Now, Shelley was the most full-minded poet of his time. He knew more about what ought to be done with the world than any of his contemporaries. That he failed to free Ireland and that the French revolution was a disaster are a reflection on other people's intelligence, not on his. It is not at all derogatory to a man's ideas that for centuries and centuries after him the world fails to come up to his teachings. If an angel is ineffectual that is not the angel's fault. Indeed a too readily effectual angel would be rather a journalist than a seer.