ea" appears to be without the mind
and at a distance off.

Not only does every _visibile_ appear to be remote, but it has a
position in external space, just as a _tangibile_ appears to be
superficial and to have a determinate position on the surface of
the body. Every _visibile_, in fact, appears (approximately) to be
situated upon a line drawn from it to the point of the retina on which
its image falls. It is referred outwards, in the general direction of
the pencil of light by which it is rendered visible, just as, in the
experiment with the stick, the _tangibile_ is referred outwards to the
end of the stick.

It is for this reason that an object, viewed with both eyes, is seen
single and not double. Two distinct images are formed, but each image
is referred to that point at which the two optic axes intersect;
consequently, the two images exactly cover one another, and appear as
completely one as any other two exactly similar superimposed images
would be. And it is for the same reason, that, if the ball of the
eye is pressed upon at any point, a spot of light appears apparently
outside the eye, and in a region exactly opposite to that in which the
pressure is made.

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 inde