If it is a question of sight, for example, and if an object changes its place before our eye, we can 'follow it with the eye' and maintain its image on the same point of the retina by appropriate movements of the eyeball.
These movements we are conscious of because they are voluntary and because they are accompanied by muscular sensations, but that does not mean that we represent them to ourselves in geometric space.
So what characterizes change of position, what distinguishes it from change of state, is that it can always be corrected in this way.
It may therefore happen that we pass from the totality of impressions A to the totality B in two different ways:
1º Involuntarily and without experiencing muscular sensations; this happens when it is the object which changes place;
2º Voluntarily and with muscular sensations; this happens when the object is motionless, but we move so that the object has relative motion with reference to us.
If this be so, the passage from the totality A to the totality B is only a change of position.
It follows from this that sight and touch could not have given us the notion of space without the aid of the 'muscular sense.'
Not only could this notion not be derived from a single sensation or even from a series of sensations, but what is more, an immobile being could never have acquired it, since, not being able to correct by his movements the effects of the changes of position of exterior objects, he would have had no reason whatever to distinguish them from changes of state. Just as little could he have acquired it if his motions had not been voluntary or were unaccompanied by any sensations.
Conditions of Compensation.—How is a like compensation possible, of such sort that two changes, otherwise independent of each other, reciprocally correct each other?