[5] The idea that time may be an active factor in causation has the mathematical significance that ‘t’ (for the system in question) must appear explicitly in the formulation of the law, and not merely as the square of a time-differential found convenient for the correlation of a standard clock with a reversible process which is being observed. A law whose mathematical formulation involves ‘t’ measured from some moment in the history of the system, gives an entirely new meaning to ‘t’, though one consistent with the properties of the reversible Newtonian differential ‘dt’. Such a law may claim to express the fact of historic, irreversible, duration, a feature in nature which is neglected by laws involving only ‘dt’ squared. [6] Einstein, _Annalen der Physik_, vol. 49, pp. 776-7, 1916. _SIXTY VOLUMES ARE NOW PUBLISHED_ TO-DAY AND TO-MORROW _Each, pott 8vo, boards, 2/6 net_ This series of books, by some of the most distinguished English thinkers, scientists, philosophers, doctors, critics, and artists, was at once recognized as a noteworthy event. Written from various points of view, one book frequently opposing the argument of another, they provide the reader with a stimulating survey of the most modern thought in many departments of life. Several volumes are devoted to the future trend of Civilization, conceived as a whole; while others deal with particular provinces. It is interesting to see in these neat little volumes, issued at a low price, the revival of a form of literature, the Pamphlet, which has been in disuse for many years. _Published by_ KEGAN PAUL, TRENCH, TRUBNER & CO., LTD. Broadway House: 68-74 Carter Lane, London, E.C.4 _FROM THE REVIEWS_ _Times Literary Supplement_: “An entertaining series of vivacious and stimulating studies of modern tendencies.” _Spectator_: “Scintillating monographs ... that very lively and courageous series.” _Observer_: “There seems no reason why the brilliant To-day and To-morrow Series should come to an end for a century of to-morrows. At first it seemed impossible for the publishers to keep up the sport through a dozen volumes, but the series already runs to more than two score. A remarkable series....” _Daily Telegraph_: “This admirable series of essays, provocative and brilliant.” _Nation_: “We are able to peer into the future by means of that brilliant series [which] will constitute a precious document upon the present time.”--_T. S. Eliot._ _Manchester Dispatch_: “The more one reads of these pamphlets, the more avid becomes the appetite. We hope the list is endless.” _Irish Statesman_: “Full of lively controversy.” _Daily Herald_: “This series has given us many monographs of brilliance and discernment.... The stylistic excellencies of this provocative series.”