l'image est le decalque 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 qualites
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 h