From Galileo to

Einstein, separating rest from uniform movement became a matter of frame of reference, or, put differently, a matter of position of the observer. A popular example of the role of the observer’s position is when we are seated in an immobile train, and the departure of another train gives us the impression that our train is moving. Galileo also invented thought experiments: if one makes the hypothesis that a theory is true and one demonstrates that reasoning based on this hypothesis leads to dead ends, then the theory is false. Performing such a thought experiment, he concluded that the speed of fall of an object is proportional to the duration of the fall and Inhibitors,research,lifescience,medical independent of the mass of the object. This was the first historical Inhibitors,research,lifescience,medical occurrence of a physical law being expressed using the parameter of time. Later on, Newton (1643-1727) asserted the reality of an absolute space and of an absolute time: “Absolute, true, and mathematical time, of itself, and from its own nature, flows equably without relation to anything external, and by another name is called Inhibitors,research,lifescience,medical duration.”17 He defined time as a succession of mathematical instants (an entity with no length). Thus, with respect to his own definition, using the name of duration is inadequate. Time according to Newton is a mathematical variable having one dimension, continuous. Only two

topological objects have this characteristic, a line and a circle. It thus follows that time is either

infinite or cyclic. Leibniz (1646-1716) was as idealistic as Plato, when he stated: “I hold space to be something merely relative, as time is, taking space to be an order of coexistences, as time is an order of successions.”18 HSP inhibitor Conversely, wrote Kant (1724-1804), one can neglect all information coming Inhibitors,research,lifescience,medical Inhibitors,research,lifescience,medical from our senses (sensitive data), but never can one leave out time and space, which are indispensable for any representation. The representation of space cannot, therefore, be empirically obtained from the relations of outer appearance. On the contrary, this outer experience is itself possible at all only through that representation.19 Time is, therefore, given a priori. In it alone is actuality of appearances possible at all. Appearances may, one and all, vanish; but, time (as the universal condition Edoxaban of their possibility) cannot itself be removed.20 The principle of causality Physicists chose the linear version of time on the basis of the principle of causality, which was first introduced by Leibniz. There are several descriptions of this principle,21 ie, the relationships between causes and effects. First, a cause necessarily precedes its effects (this precludes a cyclical time). Second, the same causes induce the same effects (and the repetition of a cause leads to the repetition of the effects, sometimes leading to cycles. I emphasize that cyclical time is not synonymous with repetition of cycles).