situated with
respect to the centres, correspond with each other, and that the
points which are dissimilarly situated, do not correspond. The truth
of this general conclusion is founded upon a a very full induction,
which is all the evidence we can have for a fact of this nature.

The next thing that seems to merit our attention, is, to inquire,
whether this correspondence between certain points of the two retinas
which is necessary to single vision, is the effect of custom, or an
original property of the human eyes.

We have a strong argument in favour of its being an original
property, from the habit we get of directing our eyes accurately to
an object; we get this habit by finding it necessary for perfect and
distinct vision; because thereby, the two images of the object
falling upon corresponding points, the eyes assist each other in
vision, and the object is seen better by both eyes together, than by
one: but when the eyes are not accurately directed, the two images of
the object fall upon points which do not correspond, whereby the
sight of the one eye disturbs that of the other. Hence it is not
unreasonable to conclude, that this correspondence between certain
points of the retina is prior to the habits we acquire in vision:
and, consequently, natural and original.

We have all acquired the habit of directing our eyes in one
particular manner, which causes single vision; now if the Author of
Nature had ordained that we should see objects single, only when our
eyes are thus directed, there is an obvious reason why all mankind
should agree in the habit of directing them in this manner; but, if
single vision were the effect of custom, any other habit of directing
the eyes would have answered the purpose; we therefore, on this
supposition, can give no reason why this particular habit should be
so universal.

Bishop Berkeley maintains a contrary opinion, and thinks that our
seeing objects single with both eyes, as well as our seeing them
erect, by inverted images, depends upon custom. In this he is
followed by Dr. Smith, who observes, that the question, why we see
objects single with both eyes, is of the same nature with this, why
we hear sounds single with both ears; and that the same answer will
serve for both; whence he concludes, that as the second of these
phenomena is the effect of custom, so also is the first. But I think,
that the questions are not so much of the same sort, as that the same
answer will