In our times, when the pace of economic, scientific and technological, social and cultural change calls to mind the relativist velocities of contemporary physics, Einstein's criterion begins to be applicable to the historical process itself; movement can be recorded (and consequently the notion of velocity acquire meaning) provided an adequate reference system is available. Where science is concerned, such systems have always existed: historians have brought out the increase in adequate knowledge in the past by comparing it with present knowledge, which then offered a system of absolute reference. But now, the theory of relativity, quantum mechanics, molecular biology and the radical transformation of standards of value in all cultural areas have made like absolutization of contemporary ideas impossible. Today ideas furnish no more than a point of departure for non-classical retrospection which, as Jaurès said, seeks the fire, not. the ashes, in the history of science. But: the fire of science, its dynamics, its movement, is inseparable from its value, its effect, its psychological ‘accompaniment.’ Contemporary science, with unprecedented dynamism, is distinguishable from classical science by its far more emotional coloring. It lays bare not only the logical collisions of knowledge but also the emotional collisions. Einstein called the history of science the ‘drama of ideas.’ A drama which sometimes turns into a tragedy. The tragic sense Lorentz was aware of in the crumbling of old conceptions is easily perceptible in the remark he made when he confessed he would have prefered to be dead before the structure of classical physics collapsed. For Erenfest the tormenting experience of a personal inability to understand the positive bases of non-classical physics was also tragic and, according to Einstein, was what drove him to suicide. Einstein's own dissatisfaction tor thirty years while he strove in the field theory alone to find a stable universal conception possessing more complete ‘internal perfection’ than the then existing solution, was again tragic. The real characteristic of the collisions of non-classical science, however, is their ambivalence: the tragic notes blend with the optimistic perception of irreversible general progress in knowledge. The collisions of knowledge and this optimistic perception together sound a very complex chord, major on the whole, but with minor harmonics.