In regard to the Cathedral, Wren gave an unhesitating opinion that nothing but an entirely new structure ought to be contemplated. This advice was not at once taken, but the fall of a part of the Cathedral where repair was being attempted gave convincing proof of the wisdom of his judgment, and in 1668 he was summoned to London from Oxford (where he was still a professor of astronomy) to advise respecting a new edifice. The taking down of the old walls--in which Wren was wonderfully ingenious in inventing devices,--lasted through part of 1668; and in 1673 Wren, who had meantime been knighted, submitted his first design for the new Cathedral,--a design which he himself wrote that he considered "antique and well studied, conformable to the best style of Greek and Roman architecture." The king greatly approved of it, and a commencement of it was actually made, but so much clerical opposition was brought to bear on account of its being different from the usual cathedral shape that Wren was reluctantly obliged to turn his thoughts in another direction; and he proceeded with several trial plans in Gothic form. One of these was accepted, and he was ordered by a royal commission, dated May, 1675, to proceed with it. The authorization was accompanied by the permission to make variations, "rather ornamental than essential"; but happily, as the whole was left to his management, he found himself able to make use of this permission without troubling himself as to the qualification as to essentials. [Illustration: THE MONUMENT LONDON] There is no concealing the point that if the design which the king's warrant authorized had been carried out unaltered, St. Paul's would, externally at least, have proved a gigantic failure. In design we may perceive that there was in Wren's mind a struggle between two ideas as respects the great central feature of the dome,--namely, that of retaining the fine and well studied internal proportions of his first design, and at the same time attaining the quality of great loftiness demanded for the external appearance. This he proposed to attain by means of a lofty spire; but before long he abandoned this attempt and adopted the idea of general height as the leading principle, by which he ultimately arrived at the unrivalled exterior of the Cathedral. Now that he was fully authorized to proceed, Wren devoted all his energies to maturing his design, and many studies are extant which show the steps by which he arrived at the final result. He had no doubt a sufficiently clear general idea in his mind's eye of what the complete structure should be, but these studies show that the details of even such essential features as the profile of the dome and the western towers were not settled until the time approached when they would be required.