the object in which the undulations originate. In like manner a light which we see is referred to its objective luminous source. But light also and in addition is reflected from, and thus reveals the presence of the whole body of our resistant environment. Hence is derived the coloured presentation of Vision to which the character of extensity attaches. Nothing similar takes place in the case of the other distantial sensations. If sonorous undulations excited vibration in every resistant object of the environment they would undoubtedly come to arrange themselves in an order resembling the extensity suggested by Vision, though the slower rate of transmission of sound would detract from the practical simultaneity in the effect which, as we have seen, largely accounts for the perception of visual extensity. The universal diffusion of sunlight is also a determining factor.

The matter becomes still clearer when we contrast the experience of vident men with what we have been able to learn of the experiences of the blind. Nowhere have we found this aspect of the question discussed with the same clearness and ability as by M. Pierre Villey in his recently published essay, Le Monde des Aveugles—Part III.

The blind man, as he remarks, requires representations in order to command his movements. We must then penetrate the mind of the blind and ascertain what are his representations. Are they, he asks, muscular images combined by temporal relations, or are they images of a spatial order? He replies without hesitation: Both, but, above all, spatial images. It is clear, he says, that the modalities of the action of the blind are explained by spatial representations. These must be derived from touch. What, then, can be the spatial representations which arise from touch? The blind, he says, are often asked, How do you figure to yourself such and such an object, a chair, a table, a triangle? M. Villey quotes Diderot as affirming that the blind cannot imagine. According to Diderot, images require colour, and colour being totally wanting to the blind the nature of their imagination was to him inconceivable. The common opinion, says M. Villey, is entirely with Diderot. It does not believe that the blind can have images of the objects around him. The photographic apparatus is awanting and the photograph cannot therefore be there.

Diderot was a sensationalist. For this school, as Villey remarks, l'image est le décalque de la sensation, and he refers not merely to Condillac the friend of Diderot but to his continuator Taine whose dictum we have already quoted.

Diderot attempts to solve the problem by maintaining that tactual sensations occupy an extended space which the blind in thought can add to or contract, and in this way equip himself with spatial conceptions.

There would, on this view, as M. Villey remarks, be a complete heterogeneity between the imagination of the blind and that of the vident. M. Villey denies this altogether. He affirms that the image of an object which the blind acquires by touch readily divests itself of the characters of tactual sensation and differs profoundly from these. He takes the example of a chair. The vident apprehends its various features simultaneously and at once; the blind, by successive tactual palpations. But he maintains that the evidence of the blind is unanimous on this point, that once formed in the mind the idea of the chair presents itself to him immediately as a whole,—the order in which its features were ascertained is not preserved, and does not require to be repeated. Indeed, the idea divests itself of the great bulk of the tactual details by which it was apprehended, whilst the muscular sensations which accompanied the act of palpation never seek to be joined with the idea. This divestiture of sensation proceeds to such an extent that there is nothing left beyond what M. Villey calls the pure form. The belief in the reality of the object he refers to its resistance. The origin of each of these is exertional. The features upon which the mind dwells, if it dwells upon them at all, are les qualités qui sont constamment utiles pour la pratique—in a word, the dynamic significance of the thing.

We may remark that much the same is true of the ideas of the vident. In ordinary Discourse we freely employ our ideas of external objects without ever attempting a detailed reproduction of the visual image. Such a reproduction would be both impracticable and unnecessary, and would involve such a sacrifice of time as to render Discourse altogether impossible. All that the Mind of the vident ordinarily grasps and utilises in his discursive employment of the idea of any physical thing is what we have ventured to call its dynamic significance. And the very careful analysis which M. Villey has made of the mental conceptions of the blind clearly shows that in their case he has reached exactly the same conclusion.

Our fundamental conceptions of the external world are therefore derived from and are built up out of the data of our exertional Activity combined with the interruptions which that Activity perpetually encounters, and in which sensations arise. It would indeed be a useful work of psychological analysis if the conditions of exertional action were carefully and systematically investigated—much more useful than most of the trifling experiments to which psychological laboratories are usually devoted.

The principal elements of such a scheme would be—

(1) The force of gravity. This force constantly operating constrains the organism to be in constant contact with the earth on which we live. But, further, it gives us the definite idea of Direction. It is from the action of gravity that we derive our distinction between Up and Down from which as a starting-point we build up our conception of tridimensional Space. And in this respect it must be remembered that as the areas of spheres are proportional to the squares of their radii it necessarily follows that gravity if it acts uniformly in tridimensional Space must vary in intensity in proportion to the square of the distance of the point of application from the centre of origin. Gravity and tridimensionality are in short necessarily connected.

(2) The same law which determines the force of gravity seems to determine also the force of cohesion, and therefore the form of material bodies. These, therefore, are necessarily subject also to tridimensionality, and in the force which generates solid form we find a second source of our elementary spatial ideas.

Such form is the expression of an obstacle to action which determines all our movements, and in which we discover those forms of the limitations of activity which we call spatial characters.

(3) Organic Dualism is a third determinant of activity, and thus also a source of spatial ideas.

The structural dualism of the human body, its right and left, its front and back, etc., furnish our activity with a set of constant forms to which its action must conform, and which necessarily also partake of, and help us to conceive of tridimensional form. It is interesting to note that this dualism characterises the organs specially adapted to serve exertional action rather than those which serve our vegetal or nutrient life.

The way in which our spatial conceptions are ever extended and built up out of the data of action is also well illustrated in the case of the blind, and to this also M. Villey devotes an interesting chapter under the title La conquête des représentations spatiales.

This is effected in their case by the high development of what we must call active touch. Just as we distinguish between