We have seen the importance of those 'internal changes' revealed to us by our muscular sensations. They may serve to characterize the various _attitudes_ of our body. Take arbitrarily as origin one of these attitudes _A_. When we pass from this initial attitude to any other attitude _B_, we feel a series of muscular sensations, and this series _S_ will define _B_. Observe, however, that we shall often regard two series _S_ and _S'_ as defining the same attitude _B_ (since the initial and final attitudes _A_ and _B_ remaining the same, the intermediary attitudes and the corresponding sensations may differ). How then shall we recognize the equivalence of these two series? Because they may serve to compensate the same external change, or more generally because, when it is a question of compensating an external change, one of the series can be replaced by the other. Among these series, we have distinguished those which of themselves alone can compensate an external change, and which we have called 'displacements.' As we can not discriminate between two displacements which are too close together, the totality of these displacements presents the characteristics of a physical continuum; experience teaches us that they are those of a physical continuum of six dimensions; but we do not yet know how many dimensions space itself has, we must first solve another question. What is a point of space? Everybody thinks he knows, but that is an illusion. What we see when we try to represent to ourselves a point of space is a black speck on white paper, a speck of chalk on a blackboard, always an object. The question should therefore be understood as follows: What do I mean when I say the object _B_ is at the same point that the object _A_ occupied just now? Or further, what criterion will enable me to apprehend this? I mean that, _although I have not budged_ (which my muscular sense tells me), my first finger which just now touched the object _A_ touches at present the object _B_. I could have used other criteria; for instance another finger or the sense of sight. But the first criterion is sufficient; I know that if it answers yes, all the other criteria will give the same response. I know it _by experience_, I can not know it _a priori_. For the same reason I say that touch can not be exercised at a distance; this is another way of enunciating the same experimental fact. And if, on the contrary, I say that sight acts at a distance, it means that the criterion furnished by sight may respond yes while the others reply no. And in fact, the object, although moved away, may form its image at the same point of the retina. Sight responds yes, the object has remained at the same point and touch answers no, because my finger which just now touched the object, at present touches it no longer. If experience had shown us that one finger may respond no when the other says yes, we should likewise say that touch acts at a distance.