brain, and they talk as readily of the moment at which a sensation
arises and of the duration of the sensation. What can they mean by
such expressions?

We have seen that sensations are not in the brain, and their
localization means only the determination of their concomitant physical
phenomena, of the corresponding brain-change. And it ought to be clear
even from what has been said above that, in determining the moment at
which a sensation arises, we are determining only the time of the
concomitant brain process. Why do we say that a sensation arises later
than the moment at which an impression is made upon the organ of sense
and earlier than the resulting movement of some group of muscles?
Because the change in the brain, to which we refer the sensation,
occurs later than the one and earlier than the other. This has a place
in real time, it belongs to that series of world changes whose
succession constitutes real time. If we ask _when_ anything happened,
we always refer to this series of changes. We try to determine its
place in the world order.

Thus, we ask: When was Julius Caesar born? We are given a year and a
day. How is the time which has elapsed since measured? By changes in
the physical world, by revolutions of the earth about the sun. We ask:
When did he conceive the plan of writing his Commentaries? If we get
an answer at all, it must be an answer of the same kind--some point in
the series of physical changes which occur in real time must be
indicated. Where else should we look for an answer? In point of fact,
we never do look elsewhere.

Again. We have distinguished between apparent space and real space
(section 34). We have seen that, when we deny that a mental image can
occupy any portion of space, we need not think of it as losing its
parts and shrivelling to a point. We may still attribute to it
apparent space; may affirm that it seems extended. Let us mark the
same distinction when we consider time. The psychologist speaks of the
duration of a sensation. Has it real duration? It is not in time at
all, and, of course, it cannot, strictly speaking, occupy a portion of
time. But we can try to measure the duration of the physical
concomitant, and call this the real duration of the sensation.

We all distinguish between the real time of mental phenomena, in the
sense indicated just above, and the apparent time. We know very well
that the one may give us no true measure of the othe