what is conveyed to us in the darkest night, it must partake of the same
properties: And as blindness and darkness afford us no ideas of
extension, it is impossible that the dark and undistinguishable distance
betwixt two bodies can ever produce that idea.

The sole difference betwixt an absolute darkness and the appearance of
two or more visible luminous objects consists, as I said, in the objects
themselves, and in the manner they affect our senses. The angles, which
the rays of light flowing from them, form with each other; the motion
that is required in the eye, in its passage from one to the other; and
the different parts of the organs, which are affected by them; these
produce the only perceptions, from which we can judge of the distance.
But as these perceptions are each of them simple and indivisible, they
can never give us the idea of extension.

We may illustrate this by considering the sense of feeling, and the
imaginary distance or interval interposed betwixt tangible or solid
objects. I suppose two cases, viz. that of a man supported in the air,
and moving his limbs to and fro, without meeting any thing tangible; and
that of a man, who feeling something tangible, leaves it, and after a
motion, of which he is sensible, perceives another tangible object; and I
then ask, wherein consists the difference betwixt these two cases? No one
will make any scruple to affirm, that it consists meerly in the
perceiving those objects, and that the sensation, which arises from the
motion, is in both cases the same: And as that sensation is not capable
of conveying to us an idea of extension, when unaccompanyed with some
other perception, it can no more give us that idea, when mixed with the
impressions of tangible objects; since that mixture produces no
alteration upon it.