[5] The best definition of the second that can be given is the following: it is the time which light takes to cover 186,000 miles in empty space and far from any strong gravitational field. This definition, the only strict definition, is further justified by the fact that there is no better means of regulating clocks than luminous or Hertzian (which have the same speed) signals. It is not less true that the time between the birth and the death of any creature, its life, will seem longer if the creature moves rapidly and fantastically relatively to the observer. In this world, where appearance is almost everything, this is not without importance, and it follows that, philosophically speaking, to move on is to last longer; but for others, not for oneself; just as others may seem to me to last longer. A striking, a profound, an unforeseen justification of the words of the sage: immobility is death! * * * * * Formerly, before the Einsteinian _hegira_, before the Relativist Era opened, everybody was convinced that the portion of _space_ occupied by an object was sufficiently and explicitly defined by its dimensions—length, breadth, and height. These are what are called the three _dimensions_ of an object; just as we speak, to use a different expression, of the longitude, latitude, and altitude of each of its points, or as we speak in astronomy of its right ascension, declination, and distance. It was quite understood that we had, in addition, to indicate the epoch, the moment, to which these data correspond. If I define the position of an aeroplane by its longitude, latitude, and altitude, these indications are only correct for a certain moment, because the aeroplane is moving relatively to the observer, and the moment also must be indicated. In this sense it has long been known that space depends upon time. But the Relativist theory shows that it depends upon time in a much more intimate and deeper manner, and that time and space are as closely connected as those twin monsters which the surgeon cannot separate without killing both. The dimensions of an object, its shape, the apparent _space_ occupied by it, depend upon its velocity: that is to say, upon the _time_ which the observer takes to traverse a certain distance relatively to the object. Here we have _space_ already depending upon _time_. In addition, the observer measures the time with a chronometer, the seconds of which are more or less accelerated according to his velocity. Hence it is impossible to define space without time. That is why we now say that time is the fourth dimension of space, or that the space in which we live has four dimensions. It is remarkable that there were able men in the past who had a more or less clear intuition of this. Thus we find Diderot, in 1777, writing in the _Encyclopédie_, in the article “Dimension”: