Considers the irreversibility of time in relation to the philosophy of history. The absence of a philosophy of history among the Greeks is attributed partly to their idea of the eternal cycle, but also to their concept of matter as uncreated, without beginning or end, of a universe without progress and thus time free from any direction. Although the idea of the cycle can be found among modern philosophers (Vico, Croce, Spengler, and Toynbee), philosophy entered a new phase with the birth of Christ, which established time in a one-way direction. Carnot-Clausius's second principle of thermodynamics determined the irreversibility of time for modern thinkers, and this was followed by Wilhelm Ostwald's establishment of this second principle as the source of all historical values. A characteristic feature of our time is the growth of its historical sense - probably because we have lived through more history than any other epoch in the evolution of mankind - which in turn explains the reawakening of the interest in the philosophy of history. The author maintains that the sufferings imposed upon individuals by "historical" events have been, in all epochs, the main motives for the development of a philosophy of history. Thus St. Augustine's 'City of God' - the first fully conscious attempt to create such a philosophy - was motivated by the conquest of Rome by the Visigoths; Machiavelli's philosophy was profoundly influenced by the invasions in Italy; Hegel's work on a philosophy of history was conceived under the influence of the Battle of Jena, and Spengler and Toynbee both produced interpretations of history under the impact of World War I and World War II.