. Of course, if we suppose that we know instinctively,
or intuitively, from the mere act of seeing, that the eye is the organ
of vision, that it forms a part of the body we behold, and is located in
the head, it requires no conjurer to prove that we _must_ have an
instinctive, or intuitive, knowledge of visible things as larger than
that organ, and, consequently, as external to it. In this case, no
process of association is necessary to account for our knowledge of the
distance of objects. That knowledge must be directly given in the very
function and exercise of vision, as every one will admit, without going
to the expense of an octavo volume to have it proved.

But we hold that no truth in mental philosophy is more incontestable
than this, that the sight originally, and of itself, furnishes us with
no knowledge of the eye, as we _now_ know that organ to exist. It does
not inform us that we have an eye at all. And here we may hazard an
observation, which, simple as it is, appears to us to be new, and not
unimportant in aiding us to unravel the mysteries of sensation; which
observation is, that, in no case whatever, does any sense inform us of
the existence of its appropriate organ, or of the relation which
subsists between that organ and its objects, but that the interposition
of some other sense[36] is invariably required to give us this
information. This truth, which we believe holds good with regard to all
the senses, is most strikingly exemplified in the case of vision, as we
shall now endeavour to illustrate.

[36] It would not be difficult to show, that as, on the one
hand, _distance_ is not involved in the original intuitions of
sight, so, on the other hand, _proximity_ is not involved in the
original intuitions of touch; but that, while it is the touch
which establishes an interval between the organ and the objects
of sight, it is the sight which establishes _no_ interval
between the organ and the objects of touch. Sight thus pays back
every fraction of the debt it has incurred to its brother sense.
This is an interesting subject, but we can only glance at it
here.

Let us begin by supposing that man is a mere "power of seeing". Under
this supposition, we must hold that the periphery of vision is one and
the same with the periphery of visible space; and the two peripheries
being identical, of course whatever objects lie within the sphere of the
one must lie within the