What I do see is that the sensations which correspond to movements in the same direction are connected in my mind by a mere _association of ideas_. It is to this association that what we call 'the sense of direction' is reducible. This feeling therefore can not be found in a single sensation. This association is extremely complex, for the contraction of the same muscle may correspond, according to the position of the limbs, to movements of very different direction. Besides, it is evidently acquired; it is, like all associations of ideas, the result of a _habit_; this habit itself results from very numerous _experiences_; without any doubt, if the education of our senses had been accomplished in a different environment, where we should have been subjected to different impressions, contrary habits would have arisen and our muscular sensations would have been associated according to other laws. CHARACTERISTICS OF PERCEPTUAL SPACE.--Thus perceptual space, under its triple form, visual, tactile and motor, is essentially different from geometric space. It is neither homogeneous, nor isotropic; one can not even say that it has three dimensions. It is often said that we 'project' into geometric space the objects of our external perception; that we 'localize' them. Has this a meaning, and if so what? Does it mean that we _represent_ to ourselves external objects in geometric space? Our representations are only the reproduction of our sensations; they can therefore be ranged only in the same frame as these, that is to say, in perceptual space. It is as impossible for us to represent to ourselves external bodies in geometric space, as it is for a painter to paint on a plane canvas objects with their three dimensions. Perceptual space is only an image of geometric space, an image altered in shape by a sort of perspective, and we can represent to ourselves objects only by bringing them under the laws of this perspective. Therefore we do not _represent_ to ourselves external bodies in geometric space, but we _reason_ on these bodies as if they were situated in geometric space. When it is said then that we 'localize' such and such an object at such and such a point of space, what does it mean? _It simply means that we represent to ourselves the movements it would be necessary to make to reach that object_; and one may not say that to represent to oneself these movements, it is necessary to project the movements themselves in space and that the notion of space must, consequently, pre-exist. When I say that we represent to ourselves these movements, I mean only that we represent to ourselves the muscular sensations which accompany them and which have no geometric character whatever, which consequently do not at all imply the preexistence of the notion of space.