15. In order to fix a time for future events, it is necessary also to take account of a circumstance inherent to the nature even of spirits.
With them time, as well as space, cannot be estimated only by aid of points of comparison or data which divides it into periods which they can count. Upon the Earth the natural division of time into days and years is marked by the rising and setting of the sun, and by the duration of the movement of translation of the Earth. The united measure of time must vary according to worlds, since the astronomical periods are different. Thus, for example, in Jupiter one day is equivalent to ten of our hours, and one year to nearly twelve of our years.
There is, then, in each world a manifest difference in computing time according to the nature of the astral revolutions which take place in it. This would make it difficult for a spirit unacquainted with our Earth to give dates. But outside of worlds these means of distinguishing time do not exist. For a spirit in space, there is for him no rising or setting of the sun marking the days, nor a periodic revolution marking the years. There is for him only duration of time and infinite space (chap. VI, from item n° 1 on). He, then, who had never come to the Earth could have no knowledge of our calculations, which besides would be useless to him. Moreover, he who had never been incarnate upon any world would have no notion of the fractions of duration of time. When a stranger spirit comes to this Earth to manifest, he cannot assign dates to events only by identifying himself with our usages, which is without doubt in his power, but that which the most frequently he judges useless to do.