John Playfair to Mary Berry, from Cambridge, 28 September 1804: 'In going into a great library, it often occurs to me to take up some remarkable book, open it by chance, and observe what comes up, as the truths that are thus casually suggested to the mind often live long in the memory. To-day, in the University library, I took up a book on the history of astronomy, called "Theatrum Cometicum," that is very scarce and very famous, and opened it to try the above experiment. The chapter that turned up was "De Causis Cometarum," and the first sentence was, "Causa cometarum maxime universalis est Deus." This truism was all I had for my pains, and is the only piece of instruction that I am likely to carry away from Cambridge. Some have perhaps gone away with less.'