Around 15 June, Darwin and FitzRoy visited the noted astronomer Sir John Herschel. In his diary, Darwin called this "the most memorable event which, for a long period, I have had the good fortune to enjoy." His zeal for science had been stirred at Cambridge by reading Herschel's book on philosophy of science, which had guided his theorising during the voyage. Their discussion is not recorded, but a few months earlier, on 20 February 1836, Herschel had written to Lyell praising his "Principles of Geology" as a work which would bring "a complete revolution in subject, by altering entirely the point of view in which it must thenceforward be contemplated." and opening a way for bold speculation on "that mystery of mysteries, the replacement of extinct species by others." Herschel himself thought catastrophic extinction and renewal "an inadequate conception of the Creator", and by analogy with other intermediate causes "the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process".</ref>