retina, in an inverted position; and though an image is pictured in
each eye, we see not _two_ objects but _one_. To philosophers, who are
even more expert in finding mysteries than in solving them, this single
vision of the erect object, from a double image of the object inverted,
has usually seemed very mysterious; and yet there is really nothing
in it at all mysterious, to any one, who has learned to consider how
much of the visual perception is referable to association. If the
light, reflected from a single object touched by us, had produced not
two merely, but two thousand separate images in our eyes, erect or
inverted, or in any intermediate degree of inclination, the visual
feeling, thus excited, however complex, would still have accompanied
the touch of a single object; and if only it had accompanied it
uniformly, the single object would have been suggested by it, precisely
in the same manner as it is now suggested by the particular visual
feeling that attends the present double inverted image. To this
supposed anomaly in the language of vision, a perfect analogy is to
be found in the most obvious cases of common language. The two words
_he conquered_ excite exactly the same notion as the single Latin
word _vicit_; and if any language were so paraphrastic as to employ
_ten_ words for the same purpose, there would be no great reason for
philosophic wonder at the unity of the notion suggested by so many
words. The two images of the single object, in the arbitrary language
of visual perception, are, as it were, two words significant of one
notion.
Whatever the simple original sensation of vision may be, then, it
is capable of being associated with other notions, so as to become
significant of them. But to what does the simple original sensation
itself amount? Is it mere colour,--or is it something more?
The universal opinion of philosophers is, that it is not _colour_
merely which it involves, but _extension_ also,--that there is a
visible figure, as well as a tangible figure--and that the visible
figure involves, in our instant original perception, superficial length
and breadth, as the tangible figure which we learn to see, involves
length, breadth, and thickness.
That it is impossible for us, at present, to separate, in the sensation
of vision, the colour from the extension, I admit; though not more
completely impossible, than it is for us to look on the thousand
feet of a meadow, and to perceive only the small inch of greenness
on our retina; and the one impossibility, as much as the other, I
conceive to arise only from intimate association, subsequent to the
original sensations of sight. Nor do I deny, that a certain part of
the retina,--which, being limited, must therefore have figure,--is
affected by the rays of light that fall on it, as a certain breadth of
nervous expanse is affected in all the other organs. I contend only,
that the perception of this limited figure of the portion of the retina
affected, does not enter into the sensation itself, more than in our
sensations of any other species, there is a perception of the nervous
breadth affected.
The immediate perception of visible figure has been assumed as
indisputable, rather than attempted to be proved,--as, before the
time of Berkeley, the immediate visual perception of distance, and of
the three dimensions of matter, was supposed, in like manner, to be
without any need of proof;--and it is, therefore, impossible to refer
to arguments on the subject. I presume, however, that the reasons,
which have led to this belief, of the immediate perception of a figure
termed visible, as distinguished from that tangible figure, which we
learn to see, are the following two,--the only reasons which I can even
imagine,--that it is absolutely impossible, in our present sensations
of sight, to separate colour from extension,--and that there are, in
fact, a certain length and breadth of the retina, on which the light
falls.
With respect to the first of these arguments, it must be admitted, by
those who contend for the immediate perception of visible figure, that
it is now impossible for us to refer to our original feelings, and that
we can speak, with absolute certainty, only of our present feelings,
or, at least, of those which we remember, as belonging to a period long
after our first sensations.
What may, or may not, have been originally separable, we cannot then,
determine. But what, even now, is the _species of extension_, which
it is impossible for us, in our visual perceptions, to separate from
colour? Is there the slightest consciousness of a perception of visible
figure, corresponding with the affected portion of the retina,--or is
not the superficial magnitude, and the only magnitude, which we connect
with colour, in any case, the very superficial magnitude which we
term tangible,--a magnitude, that does not depend on the diameter of
the retina, but is variously, greater or less, depending only on the
magnitude and distance of the external object.
The mere length and breadth, then, which we cannot separate from
colour, are not the length and breadth of the figure termed
visible,--for of the perception of these limited dimensions, we
have no consciousness,--but the length and breadth that are truly
tangible;--and there is not a single moment of visual perception, in
which the slightest evidence is afforded by our consciousness of that
difficulty of separation, with respect to the affected portion of
the expanse of the retina, on which the supposed argument, as to the
perception of visible figure, is founded.
Even though the superficial dimensions of length and breadth, connected
with colour in vision, were those of the figured retina affected, and
were necessarily limited to its small expanse, there would still be no
greater impossibility of separating the colour from mere length and
breadth in vision, than of separating it from the triple dimensions of
length, breadth, and thickness: and the argument, therefore, if it had
any force, would be equally applicable to these.
I open my eyes, in the light of day, with a wide landscape around me. I
have a sensation, or perception, of varieties of colour, and of all the
dimensions of matter. I cannot separate the colour from the length and
breadth of the trunk of a large oak before me; but equally impossible
is it for me, to separate the colour from the convexity and the
magnitude; and, from this equal impossibility, I might conclude, with
equal force, that the perception of the convexity and the magnitude is
immediate and original, as the perception of mere length and breadth.
Where all things are equal, we cannot justly deny to one what we allow
to another. He who affirms, that, in looking at a sphere, he can
separate, as elements of his sensation, the colour and the convexity,
may be allowed to use this argument of impossibility, as proof of
original connexion, in the other case. But it is only a person so
privileged by nature,--and where is such a person to be found?--who can
fairly use it.
We are able, indeed,--not while we continue to look at the sphere,
but with a sort of mental effort, afterwards to separate the colour
from the convexity, and to imagine the same colour united with any
other surface, plane or concave,--the reason of which is very evident.
Our sensation of colour has not been uniformly associated with one
species of extension, but with all its varieties; and may, therefore,
be suggested in possible coexistence with all. In all these varieties,
however, two dimensions have been constantly implied; and, therefore,
the association of colour with _these_ is complete and indissoluble. If
every surface in nature had been convex, it is by no means improbable,
that we should have found the same difficulty, in attempting to
separate colour from convexity, which we now find, in attempting to
separate it from mere length and breadth.
It is the same, in various other affections of the mind, as in our
sensations. There are feelings, which we cannot separate from other
feelings, and which, we yet know, must have been originally separate. I
might refer to the silent growth and maturity of almost every passion,
of which the mind is susceptible. But there is sufficient proof, even
in affections, which seem instantaneous. The mother, when she looks at
her babe, cannot behold it without feelings, very different from those,
which the same form and colour, in another infant, would have excited;
and yet, impossible as it is to separate, in this case, the mere visual
sensation, from that emotion of happy and instant fondness which
accompanies it, there is surely no natural connexion of the emotion,
with the mere length, and breadth, and colour.
The impossibility of separating the sensation of colour from the notion
of extension, it appears, then, is not a decisive proof of an original
connexion of these; for, if it were decisive, it would prove still
more;--and we might, from this alone, assert with equal confidence, the
original visual perception of three dimensions, as that of two, and
of the magnitude and figure, which we term _tangible_, as much as of
those, which we have chosen to term _visible_. It is surely as little
possible for us, when we open our eyes on some wide and magnificent
landscape, to separate the colour, as a mere visual sensation, from the
field, the mountain, the forest, the stream, the sky, as to separate it
from the half inch, or inch of our retina, of the perception of which
we have no consciousness in any case; and it is too much for those who
deny the immediate perception of those greater magnitudes, to urge, in
proof of the necessary original perception of this inch or half inch,
what, if valid in any respect, must establish no less the proposition
which they deny, than the proposition which they affirm.
But, it will be said, there is truly a certain figure of the part of
the retina, on which the light falls. The fact is undeniable. But
the question is, not whether such a figure exist, but whether the
perception of the figure necessarily form a part of the sensation. The
brain, and nervous system in general, are of a certain form, when they
are affected in any manner. But it does not therefore, follow,--as the
fact sufficiently shows,--that the knowledge of this form constitutes
any part of the changeful feeling of the moment. To confine ourselves,
however, to the mere senses,--it is not in the organ of sight only,
that the nervous matter is of a certain shape:--it is expanded into
some shape or other, in every organ. When the whole, or a part,
therefore, of the olfactory organ, is affected by the rays of odour,
if I may so term them, we might, with exactly the same ground for
our belief, suppose, that the knowledge of a certain extension must
accompany the fragrance, because a certain nervous expanse is, in this
case, affected, as that the notion of a certain extension must, for the
same reason, and for the same reason alone, accompany the sensation
of colour. It is because the same light, which acts upon the organ of
one person, may be made visible to another, that we conceive it more
peculiarly to be figured, as it were, on the nervous expanse, when it
is not in itself truly more figured, than the number of coexisting
particles of odour, which affect the nerve of smell. We cannot exhibit
the particles of odour, however, acting on the nostril of any one. But,
when the eye is dissected from its orbit, we can show the image of a
luminous body, distinctly formed upon the retina. _We_, the observers
of the dissected eye, have thus a clearer notion of the length and
breadth of the nervous matter affected in the one case than in the
other. But it is not in the dissected eye that vision takes place; and
as the living eye, and the living nostrils, are alike affected in more
than one physical point, we must surely admit, that in both cases, and
in both cases equally, a certain length and breadth are affected, and
that there is an olfactory figure as truly as a visible figure. The
mere visibility of the image to another person cannot alter the nature
of the organic affection itself to the sentient individual. If the
olfactory figure be not necessarily accompanied with the perception
of extension, there is no stronger reason _a priori_, to suppose that
what is termed the visible figure,--which is nothing more than a
similar affection of a nervous expanse,--should be accompanied with the
knowledge of the part of the retina affected.
These arguments, however, though they seem to me to invalidate
completely the only arguments which I can imagine to be urged in
support of our original perception of figure by the eye, are negative
only. But there is also a _positive_ argument, which seems to me
truly decisive, against the supposed necessary perception of visible
figure,--that it implies the blending of things which cannot be
blended. If the mere visual sensation of colour imply, in itself, no
figure, I can conceive it to be blended with any figure; but not so, if
it imply, in itself, a fixed definite figure, so essential to the very
sensation of the colour, that without it the colour could not for a
single moment be perceived. During the whole time, then, in which I am
gazing on a wide landscape, there is, according to the opinion of those
who contend for the necessary perception of visible figure, not colour
merely, but a certain small coloured expanse, of definite outline,
constantly perceived--since, without this, colour itself could not
be perceived; and, during all this time, there is also a notion of a
figure of a very different kind, of three dimensions, and of magnitude
almost infinitely greater, combined, not with colour merely, but with
the same coloured expanse. There must, therefore, be some possible
combination of these forms and magnitudes; since it is the colour which
we perceive that is blended with the tangible magnitudes suggested.
Now, though there are certain feelings which may coexist and unite, it
appears to me, that there are others which cannot be so blended. I may
combine, for example, my notion of a plane or convex surface, with my
notion of whiteness or blueness, hardness or softness, roughness or
smoothness; but I cannot blend my notions of these two surfaces, the
plane and the convex, as one surface, both plane and convex, more than
I can think of a whole which is less than a fraction of itself, or a
square, of which the sides are not equal, and the angles equal only
to three right angles. The same blue or white surface cannot appear
to me, then, at once plane and convex, as it must do if there be a
visible figure of one exact outline coexisting with the tactual figure
which is of a different outline; nor, even though the surface were in
both cases _plane_, can it appear to me, at the same moment, half an
inch square, and many feet square. All this must be done, however, as
often as we open our eyes, if there be truly any perception of visible
figure coexisting with the mere suggestions of touch. The visible
figure of the sphere, on which I fix my gaze, is said to be a plane of
two dimensions inseparable from colour, and this inseparable colour
must yet be combined with the sphere, which I perceive distinctly to
be convex. According to the common theory, therefore, it is at once,
to my perception, _convex_ and _plane_; and, if the sphere be a large
one, it is perceived, at the same moment, to be a sphere of many feet
in diameter, and a plane circular surface of the diameter of a quarter
of an inch. The assertion of so strange a combination of incongruities
would, indeed, require some powerful arguments to justify it; yet
is has been asserted, not merely without positive evidence, as if
not standing in need of any proof, but in absolute opposition to our
consciousness; and the only arguments which we can ever imagine to be
urged for it, are, as we have seen, of no weight,--or would tend as
much to prove the original visual perception of tangible figures, as of
the figure that is termed visible.
Is it not at least more probable, therefore, that though, like the
particles of odour when they act upon our nostrils, the rays of light
affect a portion of the retina, so as to produce on it an _image_,
which, if the eye were separated from its orbit, and its coats
dissected, might be a distinct visible figure to the eye of another
observer; this figure of the portion of the retina affected, enters
as little into the simple original sensation of sight, as the figure
of the portion of the olfactory nervous expanse, when it is affected,
enters into the sensations of smell?--and that, when the simple
affection of sight is blended with the ideas of suggestion, in what are
termed the acquired perceptions of vision,--as, for example, in the
perception of a sphere,--it is colour only which is blended with the
large convexity, and not a small coloured plane?--which small coloured
plane being necessarily limited in extent and form, so as never to be
larger than the retina itself, cannot blend with various forms and
magnitudes, and which, if it could even be supposed to constitute a
_part_ of the convexity of a sphere perceived by us, still could not
diffuse its own limited and inseparable colour over the whole magnitude
of the sphere.
I have stated to you my own opinion with respect to visible figure,--an
opinion, which to myself, I confess, appears almost certain, or, at
least, far more probable than the opinion generally entertained, that
has no evidence in our consciousness at any one moment of vision to
support it. But on subjects of this kind, which are in themselves so
very subtile, and, therefore, so liable to error, I must beg you, at
all times, and especially when the opposite sentiment has the authority
of general belief, to consider any opinion, which I may submit to you,
as offered more to your _reflection_, than for your passive adoption
of it. If I wish you,--reverently, indeed, but still freely,--to weigh
the evidence of doctrines of philosophy, which are sanctioned even by
the greatest names of every age, I must wish you still more, because it
will be still more your duty, to weigh well the evidence of opinions
that come to you, with no other authority than that of _one_ very
fallible individual.
In looking back on the senses which we have been considering, what
a boundless field do we seem already to have been endeavouring to
traverse? and how admirable would the mind have been, even though it
had been capable of no other office than that of representing, in the
union of all its sensations, as in a living mirror of the universe, the
splendid conceptions of the great Being who formed it; or, rather of
creating a-new in itself, that very universe which it represents and
admires?
Such is the power of the senses;--of
----“senses, that inherit earth and heavens,
Enjoy the various riches Nature yields;--
Far nobler, _give_ the riches they enjoy;
Give taste to fruits, and harmony to groves,
Their radiant beams to gold, and gold's bright fire;
Take in at once the landscape of the world,
At a small inlet, which a grain might close,
And half create the wonderous world they see.
But for the magic organ's powerful charm,
Earth were a rude, uncoloured chaos still;--
Like Milton's Eve, when gazing on the lake,
Man makes the matchless image, man admires.”[125]
FOOTNOTES:
[122] Judicium Paridis, v. 146--158. Ap. Mus. Anglican, vol. II. p.
274. EDIT. 1741.
[123] Inquiry into the Human Mind, &c. c. 6. sect. 1.
[124] Reid's Inquiry into the Human Mind, &c. c. 6. sect. 2.
[125] Young's Night Thoughts, VI. v. 420-427, 429-430, and 435-436.
LECTURE XXX.
HISTORY OF OPINIONS REGARDING PERCEPTION.
Gentlemen, in my last Lecture, I brought to a conclusion my remarks
on Vision, with an inquiry into the justness of the universal belief,
that, in the perception of objects by this sense, there are two
modifications of extension, a visible as well as a tangible _figure_;
the one originally and immediately perceived by the eye, the other
suggested by former experience. I stated, at considerable length, some
arguments which induce me to believe, in opposition to the universal
doctrine,--that, in what are termed the acquired perceptions of
sight, there is not this union of two separate figures of different
dimensions, which cannot be combined with each other, more than the
mathematical conceptions of a square and a circle can be combined in
the conception of one simple figure; that the original sensations of
colour, though, like the sensations of smell or taste, and every other
species of sensation, arising from affections of definite portions
of nervous substance, do not involve the perception of this definite
outline, more than mere fragrance or sweetness, but that the colour is
perceived by us as _figured_, only in consequence of being blended by
intimate associations with the feelings commonly ascribed to touch.
Philosophers, indeed, have admitted, or, at least, must admit, that we
have no consciousness of that which they yet suppose to be constantly
taking place, and that the _only figure_ which does truly seem to
us, in vision, to be combined with colour, is that which they term
tangible,--that, for example, we cannot look at a coloured sphere, of
four feet diameter, without perceiving a coloured figure, which is that
of a sphere four feet in diameter, and not a plain circular surface
of the diameter of half an inch; yet, though we have no consciousness
of perceiving any such small coloured circle, and have no reason to
believe that such a perception takes place, they still contend, without
any evidence whatever, that we see at every moment what we do not
remember to have ever seen.
After our very full discussion of the general phenomena of
perception,--as common to all our senses, and as peculiarly modified in
the different tribes of our sensations,--I might now quit a subject,
to which its primary interest as the origin of our knowledge, has
led me to pay, perhaps, a disproportionate attention. But besides
the theories, to the consideration of which our general inquiry has
incidentally led us, there are some hypothetical opinions on the
subject, of which it is necessary that you should know at least the
outline,--not because they throw any real light on the phenomena of
perception, but because, extravagantly hypothetical as they are,
they are yet the opinions of philosophers, whose eminence, in other
respects, renders indispensable some slight knowledge even of their
very errors.
In reviewing these hypotheses, it will be necessary to call your
attention to that doctrine of causation, which I before illustrated
at great length, and which I trust, therefore, I may safely take for
granted that you have not forgotten.