When the eyes are open, we see; when the ears are open, we hear; when the hand is laid on things, we feel.  How do we know that we are experiencing sensations?  The setting tells us that.  The experience in question is given together with an experience of the body.  This is concomitance of the mental and the physical as it appears in the experience of us all; and from such experiences as these the philosopher who speaks of the concomitance of physical and mental phenomena must draw the whole meaning of the word.

Let us here sharpen a little the distinction between sensations and things.  Standing at some distance from the tree, I see an apple fall to the ground.  Were I only half as far away, my experience would not be exactly the same—­I should have somewhat different sensations.  As we have seen (section 17), the apparent sizes of things vary as we move, and this means that the quantity of sensation, when I observe the apple from a nearer point, is greater.  The man of science tells me that the image which the object looked at projects upon the retina of the eye grows larger as we approach objects.  The thing, then, may remain unchanged; our sensations will vary according to the impression which is made upon our body.

Again.  When I have learned something of physics, I am ready to admit that, although light travels with almost inconceivable rapidity, still, its journey through space does take time.  Hence the impression made upon my eye by the falling apple is not simultaneous with the fall itself; and if I stand far away it is made a little later than when I am near.  In the case in point the difference is so slight as to pass unnoticed, but there are cases in which it seems apparent even to the unlearned that sensations arise later than the occurrences of which we take them to be the report.

Thus, I stand on a hill and watch a laborer striking with his sledge upon the distant railway.  I hear the sound of the blow while I see his tool raised above his head.  I account for this by saying that it has taken some time for the sound-waves to reach my ear, and I regard my sensation as arising only when this has been accomplished.

But this conclusion is not judged sufficiently accurate by the man of science.  The investigations of the physiologist and the psychologist have revealed that the brain holds a peculiar place in the economy of the body.  If the nerve which connects the sense organ with the brain be severed, the sensation does not arise.  Injuries to the brain affect the mental life as injuries to other parts of the body do not.  Hence, it is concluded that, to get the real time of the emergence of a sensation, we must not inquire merely when an impression was made upon the organ of sense, but must determine when the message sent along the nerve has reached some part of the brain.  The resulting brain change is regarded as the true concomitant of the sensation.  If there is a brain change of a certain kind, there is the corresponding sensation.  It need hardly be said that no one knows as yet much about the brain motions which are supposed to be concomitants of sensations, although a good deal is said about them.