But he who speaks thus, forgets what has been said above about the uniqueness of the relation.  In the objective order of our experiences, in the external world, we can distinguish between antecedents and consequents, between causes and their effects.  The causes and their effects belong to the one order, they stand in the same series.  The relation of the physical to the mental is, as we have seen, a different relation.  Hence, the parallelist seems justified in objecting to the assimilation of the two.  He prefers the word “concomitance,” just because it marks the difference.  He does not mean to indicate that the relation is any the less uniform or dependable when he denies that it is causal.

38.  IN WHAT SENSE MENTAL PHENOMENA HAVE A TIME AND PLACE.—­We have seen in Chapters VI and VII what space and time—­real space and time—­are.  They are the plan of the real external world and its changes; they are aspects of the objective order of experience.

To this order no mental phenomenon can belong.  It cannot, as we have seen (section 35), occupy any portion of space or even have a location in space.  It is equally true that no series of mental changes can occupy any portion of time, real time, or even fill a single moment in the stream of time.  There are many persons to whom this latter statement will seem difficult of acceptance; but the relation of mental phenomena to space and to time is of the same sort, and we can consider the two together.

Psychologists speak unhesitatingly of the localization of sensations in the 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.