But in the course of my remarks on the phenomena of change in a two-dimensional world, I pointed out that it is possible to integrate an infinite number of three-spatial simultaneities into a four-dimensional whole.
The introduction of this concept seems to me calculated to modify the whole aspect of the question.
For, by its light, we see that all the three-spatial simultaneities by which we mark time can exist at once.
They can do so because the arrangement of material particles which constitutes a given simultaneity may be regarded, if we so wish, as a thin section of a four dimensional solid.
We can say, then, that there are two sorts of time.
First there is ordinary Physical "time" which is measured by the recurrence of three-spatial simultaneities and this, if we choose, may be regarded as produced by the passage across our space of something which has extension in four dimensions.
Secondly, there is what I am inclined to call Subjective time, consisting of changes in Psychic states; and which may be regarded, provisionally, as being perceived by virtue of changes in "objects," including the vehicles of our own consciousnesses, in space of four dimensions, or, at any rate, in space of a dimensionality higher than three.
I do not mean the foregoing remarks to be taken too literally for I do not regard three-dimensional change as produced by the passage across our space of actual four-dimensional solids. This seems to me to be altogether too crude an idea and was only introduced to bring out my point that three-dimensional change is capable of expression in terms of four space.
Whether it is solely a phenomenon of consciousness or whether there may be something in the nature of four-dimensional "lines of force" which cut three-dimensional space and determine material distributions I am not at all prepared even to surmise.