you to start from your original position, and moving in the direction in which the procession was passing, overtake it at some lower street corner, you could witness the thing you had already seen. This would represent post-vision—clairvoyance of the past.
A higher type of clairvoyance would be represented by the sweep of vision possible from a balloon. From that place of vantage the procession would be seen, not as a sequence, but simultaneously, and could be traced from its formation to its dispersal. Past, present and future would be merged in one.
It is true that this explanation raises more questions than it answers: to account in this way for a marvel, a greater marvel must be imagined—that of transport out of one's own "space." The whole subject bristles with difficulties, not the least of which is that even to conceive of such a thing as prevision all our old ideas about time must be recast. This is being done in the Principle of Relativity, a subject which may appropriately engage our attention next.