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. But of course our
touchless man would be devoid of any notion of resistance; and hence
space, for him, would be altogether geometrical and devoid of body.

And here another curious consideration arises, what likeness, if
any, would there be between the visual space of the one man, and the
tangible space of the other?

Berkeley, as we have seen (in the eighth proposition), declares that
there is no likeness between the ideas given by sight and those given
by touch; and one cannot but agree with him, so long as the term ideas
is restricted to mere sensations. Obviously, there is no more likeness
between the feel of a surface and the colour of it, than there is
between its colour and its smell. All simple sensations, derived
from different senses, are incommensurable with one another, and only
gradations of their own intensity are comparable. And thus so far as