Space is a three-dimensional continuum. By this we mean that it is possible to describe the position of a point (at rest) by means of three numbers (co-ordinates) _x, y, z_, and that there is an indefinite number of points in the neighbourhood of this one, the position of which can be described by co-ordinates such as _x1, y1, z1_, which may be as near as we choose to the respective values of the co-ordinates _x, y, z_, of the first point. In virtue of the latter property we speak of a “continuum,” and owing to the fact that there are three co-ordinates we speak of it as being “three-dimensional.” Similarly, the world of physical phenomena which was briefly called “world” by Minkowski is naturally four dimensional in the space-time sense. For it is composed of individual events, each of which is described by four numbers, namely, three space co-ordinates _x, y, z_, and a time co-ordinate, the time value _t_. The “world” is in this sense also a continuum; for to every event there are as many “neighbouring” events (realised or at least thinkable) as we care to choose, the co-ordinates _x1, y1, z1, t1_ of which differ by an indefinitely small amount from those of the event _x, y, z, t_ originally considered. That we have not been accustomed to regard the world in this sense as a four-dimensional continuum is due to the fact that in physics, before the advent of the theory of relativity, time played a different and more independent rôle, as compared with the space coordinates. It is for this reason that we have been in the habit of treating time as an independent continuum. As a matter of fact, according to classical mechanics, time is absolute, _i.e._ it is independent of the position and the condition of motion of the system of co-ordinates. We see this expressed in the last equation of the Galileian transformation (_t′_ = _t_). The four-dimensional mode of consideration of the “world” is natural on the theory of relativity, since according to this theory time is robbed of its independence. This is shown by the fourth equation of the Lorentz transformation: image030 Moreover, according to this equation the time difference Δ_t′_ of two events with respect to _K′_ does not in general vanish, even when the time difference Δ_t_ of the same events with reference to _K_ vanishes. Pure “space-distance” of two events with respect to _K_ results in “time-distance ” of the same events with respect to _K_. But the discovery of Minkowski, which was of importance for the formal development of the theory of relativity, does not lie here. It is to be found rather in the fact of his recognition that the four-dimensional space-time continuum of the theory of relativity, in its most essential formal properties, shows a pronounced relationship to the three-dimensional continuum of Euclidean geometrical space.[14] In order to give due prominence to this relationship, however, we must replace the usual time co-ordinate t by an imaginary magnitude image031