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 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 the primary facts of sensation go, visual figure and tactile figure, visual magnitude and tactile magnitude, visual motion and tactile motion, are truly unlike, and have no common term.  But when Berkeley goes further than this, and declares that there are no “ideas” common to the “ideas” of touch and those of sight, it appears to me that he has fallen into a great error, and one which is the chief source of his paradoxes about geometry.