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.