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.