One point in my last chapter, that on the Results and Limits of Scientific Method, has been criticised by Professor W. K. Clifford in his lecture[20] on “The First and the Last Catastrophe.” In vol. ii. p. 438 of the first edition (p. 744 of this edition) I referred to certain inferences drawn by eminent physicists as to a limit to the antiquity of the present order of things. “According to Sir W. Thomson’s deductions from Fourier’s *theory of heat*, we can trace down the dissipation of heat by conduction and radiation to an infinitely distant time when all things will be uniformly cold. But we cannot similarly trace the Heat-history of the Universe to an infinite distance in the past. For a certain negative value of the time, the formulæ give impossible values, indicating that there was some initial distribution of heat which could not have resulted, according to known laws of nature, from any previous distribution.” [20] *Fortnightly Review*, New Series, April 1875, p. 480. Lecture reprinted by the Sunday Lecture Society, p. 24. Now according to Professor Clifford I have here misstated Thomson’s results. “It is not according to the known laws of nature, it is according to the known laws of conduction of heat, that Sir William Thomson is speaking.... All these physical writers, knowing what they were writing about, simply drew such conclusions from the facts which were before them as could be reasonably drawn. They say, here is a state of things which could not have been produced by the circumstances we are at present investigating. Then your speculator comes, he reads a sentence and says, ‘Here is an opportunity for me to have my fling.’ And he has his fling, and makes a purely baseless theory about the necessary origin of the present order of nature at some definite point of time, which might be calculated.”