Gall did not foresee that a physiological experiment (and a very sure one it is) would one day demonstrate that the sense receives the impression but does not perceive it, and that, consequently, it is endowed neither with conscience nor reminiscence, &c.
When the cerebral lobes or hemispheres[97] are removed from an animal, the animal immediately loses its sight.
And yet nothing, as regards the eyes themselves, has been changed; objects continue to be depicted upon the retina, the iris retains its contractility, and the optic nerve its excitability. The retina continues to be sensible of light, for the iris contracts or dilates according as the light admitted to it is more or less intense.
No change has taken place as to the structure of the eye, and yet the animal does not see! Therefore it is not the eye that perceives, nor is it the eye that sees.[98]
The eye does not see; it is the understanding that sees by means of the eyes.[99]
When Gall concludes from the independence of the external senses to the independence of the faculties of the soul, he confounds, as to the sense itself, two things that are essentially distinct, impression and perception. Impression is multiple; perception is single.
When the hemispheres are removed, the animal instantly loses its perception; it no longer sees nor hears,[100] &c. notwithstanding all the organs of the senses, the eye, the ear, &c. subsist, and the impressions take place.
Therefore the principle that perceives is one. Lost for one sense, it is lost for all the senses. And if it be one for the external senses, how can it be other than one for the faculties of the soul?
Gall therefore cannot suppose the existence of several distinct principles for the faculties of the soul, otherwise than because he supposes several distinct principles for the perceptions; and he only supposes several principles for the perceptions because he confounds impression with perception. The whole of his psycology arises from a mistake; and the whole of his anatomy is constructed for the sake of his psycology.
In psycology he endeavours to prove that the faculties of the soul are merely internal senses; in anatomy, he endeavours to prove that the organs of the faculties of the soul only repeat and reproduce the organs of the external senses.