But while it seems to me that there is no reason to doubt that the
extradition of sensation is more complete in the case of the eye than
in that of the skin, and that corporeal distinctness, and hence space,
are directly suggested by vision, it is another, and a much more
difficult question, whether the notion of geometrical solidity is
attainable by pure vision; that is to say, by a single eye, all the
parts of which are immoveable. However this may be, for an absolutely
fixed eye, I conceive there can be no doubt in the case of an eye that
is moveable and capable of adjustment. For, with the moveable eye,
the muscular sense comes into play in exactly the same way as with the
moveable hand; and the notion of change of place, _plus_ the sense of
effort, gives rise to a conception of visual space, which runs exactly
parallel with that of tangible space. When two moveable eyes are
present, the notion of space of three dimensions is obtained in the
same way as it is by the two hands, but with, much greater precision.
And if, to take a case similar to one already assumed, we suppose a
man deprived of every sense except vision, and of all motion except
that of his eyes, it surely cannot be doubted that he would have a
perfect conception of space; and indeed a much more perfect conception
than he who possessed touch alone without vision.
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