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
(q_1, q_2, q_3, q_4) determines an event-particle, and (q_1, q_2, q_3)
determines a point and q_4 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.

So far in speaking of space we have been talking of the timeless space
of physical science, namely, of our concept of eternal space in which
the world adventures. But the space which we see as we look about is
instantaneous space. Thus if our natural perceptions are adjustable to
the p-system of measurements we see instantaneously all the
event-particles at some definite time p_4, and observe a succession
of such spaces as time moves on. The timeless space is achieved by
stringing together all these instantaneous spaces. The points of an
instantaneous space are event-particles, and the points of an eternal
space are strings of event-particles occurring in succession. But the
man on Mars will never perceive the same instantaneous spaces as the man
on the earth. This system of instantaneous spaces will cut across the
earth-man's system. For the earth-man there is one instantaneous space
which is the instantaneous present, there are the past spaces and the
future spaces. But the present space of the man on Mars cuts across the
present space of the man on the earth. So that of the event-particles
which the earth-man thinks of as happening now in the present, the man
on Mars thinks that some are already past and are ancient history, that
others are in the future, and others are in the immediate present. This
break-down in the neat conception of a past, a present, and a future is
a serious paradox. I call two event-particles which on some or other
system of measurement are in the same instantaneous space 'co-present'
event-particles. Then it is possible that A and B may be co-present,
and that A and C may be co-present, but that B and C may not be
co-present. For example, at some inconceivable distance from us there
are events co-present with us now and also co-present with the birth of
Queen Victoria. If A and B are co-present there will be some systems
in which