Suppose you are standing on a street corner, watching a procession pass.  You see the pageant as a sequence of objects and individuals appearing into view near by and suddenly, and disappearing in the same manner.  This would represent our ordinary waking consciousness of what goes on in the world round about.  Now imagine that you walk up the street in a direction opposite to that in which the procession is moving.  You then rapidly pass in review a portion of the procession which had not yet arrived at the point you were a few moments before.  This would correspond to the seeing of something before it “happened,” and would represent the positive aspect of clairvoyance in time—­prevision.  Were 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.

V CURVED TIME

TIME FROM THE STANDPOINT OF EXPERIMENT AND OF CONSCIOUS EXPERIENCE

In some moment of “sudden light” what one of us has not been able to say, with Rossetti,

    “I have been here before,
    But when or how I cannot tell.”

Are such strange hauntings of our House of Life due to the cyclic return of time?  Perhaps,—­but what is time?

Suppose some one should ask you, “What is an hour?” Your answer might be, “It is the interval marked off by the clock-hand between 1 and 2.”  “But what if your clock is running down or speeding up?” To this you would probably reply, “The clock is set and corrected by the earth, the sun and the stars, which are constant in their movements.” But they are not.  The earth is known to be running slow, by reason of tide friction, and this is likely to continue until it will revolve on its axis, not once a day, but once a year, presenting always the same face to the sun.

We can only measure time by uniform motion.  Observe the vicious circle.  Uniform motion means the covering of equal spaces in equal times.  But how are we to determine our equal times?  Ultimately we have no other criterion save the uniform motion of the clock-hand or the star dial.  The very expressions, “uniform motion,” “equal times,” beg the whole question of the nature of time.