2. We ascertain likewise that the same external change may be corrected by two internal changes corresponding to different muscular sensations. Here again I can ascertain this without knowing geometry; and I have no need of anything else; but I proceed to give the explanation of the fact, employing geometrical language. To go from the position _A_ to the position _B_ I may take several routes. To the first of these routes will correspond a series _S_ of muscular sensations; to a second route will correspond another series _S''_, of muscular sensations which generally will be completely different, since other muscles will be used. How am I led to regard these two series _S_ and _S''_ as corresponding to the same displacement _AB_? It is because these two series are capable of correcting the same external change. Apart from that, they have nothing in common. Let us now consider two external changes: [alpha] and [beta], which shall be, for instance, the rotation of a sphere half blue, half red, and that of a sphere half yellow, half green; these two changes have nothing in common, since the one is for us the passing of blue into red and the other the passing of yellow into green. Consider, on the other hand, two series of internal changes _S_ and _S''_; like the others, they will have nothing in common. And yet I say that [alpha] and [beta] correspond to the same displacement, and that _S_ and _S''_ correspond also to the same displacement. Why? Simply because _S_ can correct [alpha] as well as [beta] and because [alpha] can be corrected by _S''_ as well as by _S_. And then a question suggests itself: If I have ascertained that _S_ corrects [alpha] and [beta] and that _S''_ corrects [alpha], am I certain that _S''_ likewise corrects [beta]? Experiment alone can teach us whether this law is verified. If it were not verified, at least approximately, there would be no geometry, there would be no space, because we should have no more interest in classifying the internal and external changes as I have just done, and, for instance, in distinguishing changes of state from changes of position. It is interesting to see what has been the rôle of experience in all this. It has shown me that a certain law is approximately verified. It has not told me _how_ space is, and that it satisfies the condition in question. I knew, in fact, before all experience, that space satisfied this condition or that it would not be; nor have I any right to say that experience told me that geometry is possible; I very well see that geometry is possible, since it does not imply contradiction; experience only tells me that geometry is useful. 6. _Visual Space_