If (p₁, p₂, p₃, p₄) be a set of measurements of this system, then the event-particle which is thus determined will be said to have p₁, p₂, p₃, p₄ as its co-ordinates in this system of measurement. Suppose that we name it the p-system of measurement. Then in the same p-system by properly varying (p₁, p₂, p₃, p₄) every event-particle that has been, or will be, or instantaneously is now, can be indicated. Furthermore, according to any system of measurement that is natural to us, three of the co-ordinates will be measurements of space and one will be a measurement of time. Let us always take the last co-ordinate to represent the time-measurement. Then we should naturally say that (p₁, p₂, p₃) determined a point in space and that the event-particle happened at that point at the time p₄. But we must not make the mistake of thinking that there is a space in addition to the space-time manifold. That manifold is all that there is for the determination of the meaning of space and time. We have got to determine the meaning of a space-point in terms of the event-particles of the four-dimensional manifold. There is only one way to do this. Note that if we vary the time and take times with the same three space co-ordinates, then the event-particles, thus indicated, are all at the same point. But seeing that there is nothing else except the event-particles, this can only mean that the point (p₁, p₂, p₃) of the space in the p-system is merely the collection of event-particles (p₁, p₂, p₃, [p₄]), where p₄ is varied and (p₁, p₂, p₃) is kept fixed. It is rather disconcerting to find that a point in space is not a simple entity; but it is a conclusion which follows immediately from the relative theory of space. Furthermore the inhabitant of Mars determines event-particles by another system of measurements. Call his system the q-system. According to him (q₁, q₂, q₃, q₄) determines an event-particle, and (q₁, q₂, q₃) determines a point and q₄ a time. But the collection of event-particles which he thinks of as a point is entirely different from any such collection which the man on earth thinks of as a point. Thus the q-space for the man on Mars is quite different from the p-space for the land-surveyor on earth.