But literature stood to him as the great exponent of all that was permanent in the human spirit. “There is much,” he says, “that is deciduous in books, but all that gives them a title to rank as literature in the highest sense is perennial. Their vitality is the vitality not of one or another blood or tongue, but of human nature; their truth is not topical and transitory, but of universal acceptation; and thus all great authors seem the coevals not only of each other, but of whoever reads them, growing wiser with him as he grows wise, and unlocking to him one secret after another as his own life and experience give him the key, but on no other condition.”[21] It was with this principle determining his choice that he proceeded with more or less conscious assembling to discourse on Carlyle, Emerson, Lessing, Rousseau, Shakespeare, Dryden, Chaucer, Pope, Milton, Dante, Spenser, and Wordsworth, as well as to write in many detached passages on the genius of Goethe. Later he returned to the same general field, and besides revising his judgment on some of these topics, treated also with more or less fulness of Gray, Cervantes, Fielding, and Coleridge, while any one who consults the elaborate index to his prose writings will readily see how many other authors who belong in the great ranks have been drawn upon for illustration of the one great theme. To his reading of all this literature he brought the touchstone of his own life and experience. In this word “experience,” moreover, must be included his own highest experiments. His poetry, for the most part, as we have already seen, does not have its roots in other literature; it springs from that life which he held in common with those whom he reverenced for their own acts of literary creation. He quotes the recommendation of a friend that he should read poetry, feed himself on bee bread so that he might get into the mood of writing poetry; but, though all his life long Lowell fed, as by the most natural appetite, on poetry and other forms of imaginative literature, his own poetry is not bookish, nor does it borrow in form or phrase. Even when most impressionable in his youth, the influence upon him of Keats and Tennyson was more obvious than that of Shakespeare or Marlowe, only because, eschewing the imitative, his verse took the color of his generation. The likenesses were always general, and when he essayed forms of verse most rigid in their historical development, as the sonnet and the ode, he simply obeyed the law as his great progenitors had done, finding his freedom within the law, and not in outbreaks and protests. The conscious intention to be original, he himself says, seldom leads to anything better than extravagance; and there is a passage in his paper on Chaucer which sums up a large part of his literary philosophy.[22]